stm32: add stm32l431 ec in chip/stm32 : uart
stm32: add stm32l431 ec in chip/stm32 : uart

stm32l431 related driver: uart
The stm32l476g-eval is the only board which would be impacted.

BRANCH=main
BUG=b:188117811
TEST=make buildall

Signed-off-by: Bossen WU <bdf0d5f9d1715eb5414024c4670c571841ae5f36@stmicro.corp-partner.google.com>
Change-Id: I861d1cf38430d6b1b5d7c09bd565d727961a4128
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2975168
Reviewed-by: Ting Shen <c671d3efec3d7e7e285f3788bf0d1ba4c8605b78@chromium.org>
Reviewed-by: Eric Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec