AT91: at91sam9g20ek: merge 2mmc version in one board

The board-sam9g20ek-2slot-mmc.c was a revision of the at91sam9g20ek
since board revision C. It contains 2 sd/mmc slots.

This merge keep the support of the old machine ID
MACH_AT91SAM9G20EK_2MMC for backward compatibility.

Now we use the ATAG to pass the hardware functionality to kernel
with this board revision encoding
 bit 0:
     0 => 1 sd/mmc slot
     1 => 2 sd/mmc slots connectors (board from revision C)

system_rev tested on Barebox commit d8f3ee103a9f4bd

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