soc/it8xxx2: add support for raising EC bus to 24MHz

This change was made to reduce read/write EC registers latency.
Without enabling CONFIG_SOC_IT8XXX2_EC_BUS_24MHZ:
- Read EC register 64 times takes 80us latency.
- Write EC register 64 times takes 60us latency.
With enabling CONFIG_SOC_IT8XXX2_EC_BUS_24MHZ:
- Read EC register 64 times takes 40us latency.
- Write EC register 64 times takes 30us latency.

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