MA-6537 tty: serial: imx: enable ipg clock for register save/restore in megafix power off

In megafix power off, the driver save necessary registers in suspend, and resotre the
registers in resume, but access registers need ipg clock gate on, so the patch make sure
ipg clock gate is on before save/restore the uart registers.

Signed-off-by: Fugang Duan <B38611@freescale.com>
(cherry picked from commit: b72847043d629fe2bc152201485f4da6bb16fc89)