boards: arm: xiao_ble: use spi2 instead of spi0

There are 3 SPI interfaces (spi[0-2]) while only 2 I2C
interfaces (i2c[0-1]). Configure spi2 instead of spi0, so that i2c0 (which
conflicts with spi0) can be used for IMU (on XIAO BLE SENSE board variant).

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>