spi: atmel: remove warning when !CONFIG_PM_SLEEP

When CONFIG_PM is defined but not CONFIG_PM_SLEEP (this happens when
CONFIG_SUSPEND is not defined), there is the following warning:

drivers/spi/spi-atmel.c:1723:12: warning: ‘atmel_spi_suspend’ defined but not used [-Wunused-function]
drivers/spi/spi-atmel.c:1741:12: warning: ‘atmel_spi_resume’ defined but not used [-Wunused-function]

Enclose both atmel_spi_suspend and atmel_spi_resume in #ifdef
CONFIG_PM_SLEEP/#endif to solve that.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>