mgmt: ec_host_cmd: add SPI SMT32 backend

Add support for SPI host command backend for STM32 chips family.

Unfortunately, the current SPI API can't be used to handle the host
commands communication. The main issues are unknown command size sent
by the host(the SPI transaction sends/receives specific number of bytes)
and need to constant sending status byte(the SPI module is enabled and
disabled per transaction). Thus the SPI backend includes basic SPI STM32
driver adjusted to host command specification.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>