spi: sifive: add PM callbacks to support suspend/resume

The patch has been tested on Unmatched using pm_test. The Unmatched board
uses SD over SPI and it was tested by initiating S2RAM cycles for all
devices while reading/writing files at the same time. We found no dropped
connection to the card or corrupted filesystem during test cycles.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Greentime Hu <greentime.hu@sifive.com>
Link: https://lore.kernel.org/r/20220610074459.3261383-2-andy.chiu@sifive.com


Signed-off-by: Mark Brown <broonie@kernel.org>