soc: it8xxx2: add soc variants

Currently there are two soc variants (IT81202BX and IT81302BX) on
it8xxx2 series. The IT81202BX is 128-pin package (GPIO K and L groups
aren't bonding with pad).
This makes soc variant configurable and apply corresponding configuration
for a soc.

Signed-off-by: Dino Li <Dino.Li@ite.com.tw>