spi/atmel: improve the system suspend/resume functions implementation

To make it cleaner, the system suspend/resume directly call
the runtime suspend/resume functions
and remove the wapper of CONFIG_PM_RUNTIME, CONFIG_PM_SLEEP.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Kevin Hilman <khilman@linaro.org>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>