Add Seeed XIAO nRF54L15 Sense + Wio-SX1262 variant with GPS and IMU support

[hackengineer]

New variant for the Seeed Studio XIAO nRF54L15 Sense stacked with the
Wio-SX1262 for XIAO (header pinout, standalone SKU 113010003) and,
optionally, the L76K GNSS Module for Seeed Studio XIAO. Registered as
PRIVATE_HW following the nrf54l15dk convention.

Pin map, board overlay (uart21 enabled, i2c22 freed for the radio's D4/D5,
MCUboot slot1 reclaimed for LittleFS), and hardware notes are in the
variant's README.

Platform work included, all device-tree- or arch-guarded so existing
targets (including nrf54l15dk) are unaffected:

• Real Serial1 in the nRF54L15 Arduino shim: bound to Zephyr uart21
  (XIAO D6/D7) with interrupt-driven RX into a ring buffer, runtime baud
  reconfiguration for GPS probing, and poll-out TX. Console instances keep
  the existing printk behavior; the device binding is guarded by
  DT_NODE_HAS_STATUS(..., okay), so builds that leave uart21 disabled
  compile identically.
• ARCH_NRF54L15 added to GPS.cpp's serial-port selection branches
  (previously fell through to #error Unsupported architecture!).
• GPS_SKIP_PROBE: generalizes the existing SENSECAP_INDICATOR
  stream-only handling for variants whose GNSS command channel is
  unavailable. On this stack the L76K shield's reset line is shared with
  the radio's RESET on D2, so probe commands are unreliable while the NMEA
  stream itself is fine — the variant parses the stream directly at the
  module's default 9600 baud.
• LSM6DS3 accelerometer (on the Sense board, i2c30 @ 0x6a) enabled for
  wake-on-motion: motion sources re-added to the build and the Adafruit
  LSM6DS driver pulled in. BMX160Sensor.cpp is included for its stub
  constructor because AccelerometerThread's BMX160 switch case is not
  include-guarded.

Verified on hardware: BLE phone-API bootstrap (encrypted fixed-PIN pairing,
bond persistence across reboots), LoRa mesh TX/RX/relay on US LongFast,
LSM6DS3 detection and init, and GPS fixes (16+ satellites, PDOP ~1.0) from
the L76K.

BLE reliability on this board depends on #11471 (sleep-clock and phone-API
fixes); this PR builds independently of it.

🤝 Attestations

• I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • Heltec (Lora32) V3
  • LilyGo T-Deck
  • LilyGo T-Beam
  • RAK WisBlock 4631
  • Seeed Studio T-1000E tracker card
  • Other (please specify below)

Tested on the target hardware (XIAO nRF54L15 Sense + Wio-SX1262 + L76K).
All shared-code changes are behind ARCH_NRF54L15, a device-tree status
guard, or the new opt-in GPS_SKIP_PROBE define, so the listed devices
compile byte-identical firmware.

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