spi: atmel: use managed resource for gpio chip select

Use the managed gpio CS pin request so that we avoid having trouble
in the cleanup code.
In fact, if module was configured with DT, cleanup code released
invalid pin.  Since resource wasn't freed, module cannot be reinserted.

This require to extract the gpio request call from the "setup" function
and call it in the appropriate probe function.

Reported-by: Alexander Morozov <linux@meltdown.ru>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>