spi: atmel: add deepest PM support to SAMA5D2

This adds deepest (Backup+Self-Refresh) PM support to the ATMEL SAMA5D2
SoC's SPI controller.

When resuming from deepest state, it is required to restore MR register
as the registers are lost since VDD core has been shut down when
entering deepest state on the SAMA5D2.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>