at91: at91sam9g20ek modify dual slot evaluation kit

at91sam9g20ek rev. C and onwards embed two SD/MMC slots. This patch modify the
previous dual slot board definition to match the official rev. C board. It also
allows the use of at91_mci SD/MMC driver in addition to the atmel-mci one.

Some pins have been re-affected from leds or Ethernet phy IRQ to the SD/MMC
slot A. This lead to a modification of those definitions.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Andrew Victor <linux@maxim.org.za>