spi: atmel-quadspi: Disable the QSPI IP at suspend()

It is safer to disable the QSPI IP at suspend, in order to avoid
possible impact of glitches on the internal FSMs. This is a theoretical
fix, there were no problems seen as of now. Tested on sama5d2 and
sam9x60 versions of the IP.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20210210135428.204134-1-tudor.ambarus@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>