drivers/i2c: it8xxx2: Allow I2C target entry power saving mode

Add I2C_TARGET_ALLOW_POWER_SAVING config. Enable this config makes I2C
target device can enter Doze/Deep doze states while the bus is idle.
Ongoing transfers will block low-power entry until they are completed,
ensuring correct communication while still reducing overall power
consumption.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>