serial: sifive: Add suspend and resume operations

If the Sifive Uart is not used as the wake up source, suspend the uart
before the system enter the suspend state to prevent it woken up by
unexpected uart interrupt. Resume the uart once the system woken up.

Signed-off-by: Nick Hu <nick.hu@sifive.com>
Reviewed-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://lore.kernel.org/r/20230815090216.2575971-1-nick.hu@sifive.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>