ITE: drivers/serial: add the UART driver for the PM callback function

IT8XXX2 uses shared ns16550.c driver which does not provide a power
management callback(pm_action_cb), so create driver to handle
IT8XXX2 specific UART features.

note: pm_action_cb(old name: pm_control_fn)

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