[ARM] 4760/1: [AT91] SPI CS0 errata on AT91RM9200

Due to errata regarding the handling of SPI CS0 on the AT91RM9200, the
atmel_spi driver drives CS0 from the SPI controller and not as a GPIO
pin.
We therefore need to configure CS0 for use by the controller

Signed-off-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>