spi: atmel: Remove platform data support

This driver is now only used through the device tree. Simplify code
by explicitly depend on device tree.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Link: https://lore.kernel.org/r/20191017141846.7523-6-gregory.clement@bootlin.com


Signed-off-by: Mark Brown <broonie@kernel.org>