ARM: at91: remove pinctrl conflict between mmc and SPI for at91sam9g20ek

These MMC and SPI buses can't be configured at the same time because they
share the same traces on the EK board.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>