Add suspend and resume hooks to save/restore the registers content
during S2RAM operation.

Also save/restore the oversampling rate register (OSAMP) as earlier
stages already tuned that register to get a precise UART clock.
Suggested-by: NAllen Yan <yanwei@marvell.com>
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>