ARM: at91/DT: fix SPI compatibility string

In previous version of SPI driver we where using different compatibility stings
for finding SPI features. We are now using the IP revision information.
So we stay with the unique compatibility string for this driver:
"atmel,at91rm9200-spi".

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Wenyou Yang <wenyou.yang@atmel.com>