[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>

arch/arm/Makefile

@@ -124,7 +124,7 @@ endif
  machine-$(CONFIG_ARCH_H720X)	   := h720x
  machine-$(CONFIG_ARCH_AAEC2000)   := aaec2000
  machine-$(CONFIG_ARCH_REALVIEW)   := realview
- machine-$(CONFIG_ARCH_AT91)	   := at91rm9200
+ machine-$(CONFIG_ARCH_AT91)	   := at91
  machine-$(CONFIG_ARCH_EP93XX)	   := ep93xx
  machine-$(CONFIG_ARCH_PNX4008)	   := pnx4008
  machine-$(CONFIG_ARCH_NETX)	   := netx