boards: arm: add Seeed XIAO BLE board

The Seeed XIAO BLE is a tiny (21 mm x 17.5 mm) Nordic Semiconductor
nRF52840 ARM Cortex-M4F development board with onboard LEDs, USB port,
QSPI flash, battery charger, and range of I/O broken out into 14 pins.

Co-authored-by: Peter Johanson <peter@peterjohanson.com>
Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>