[ARM] 5021/1: at91: buildfix for sam9263 + PM

Build fix for power management on at91sam9263:  it has two memory
controllers instead of just one, so it might have two banks of
DRAM to put into selfrefresh mode.  For now we continue to assume
only the first bank is populated.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>