driver: i2c: npcx: prevent sleep during I2C transactions

Deep Sleep mode stops SMB module clocks which could interrupt ongoing
I2C transactions, so have the I2C driver acquire a PM lock at the
beginning of a transaction and release it at the end in order to ensure
the module remains active.

Signed-off-by: Peter Marheine <pmarheine@chromium.org>
Signed-off-by: Jun Lin <CHLin56@nuvoton.com>