dts/arm/st: wl: increase Sub-GHz SPI frequency to 12MHz

The maximum supported speed according to the SX126x datasheet (I have
not found that information in the STM32WL datasheet or reference
manual). Increase the Sub-GHz SPI frequency from 1 Mhz to to 12 MHz,
which corresponds to a baud rate prescaler of 4 with a 48 MHz clock. It
also matches what is done the the STM32CubeWL package.

This reduces the time the MCU is kept running, thus reducing the global
power consumption.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>