[ARM] fix mach-at91 build breakage

The rename of the AT91 subtree from mach-at91rm9200 to mach-at91
(to accomodate at91sam926x processors) was incomplete.  It needs
this patch to be able to build again.

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