at91: vmalloc fix missing AT91_VIRT_BASE define

VMALLOC_END is defined in terms of AT91_VIRT_BASE but this needs
mach/hardware.h for it's definition.

In file included from arch/arm/mach-at91/board-usb-a926x.c:26:0:
include/linux/mm.h: In function 'is_vmalloc_addr':
include/linux/mm.h:305:41: error: 'AT91_VIRT_BASE' undeclared (first use in this function)
include/linux/mm.h:305:41: note: each undeclared identifier is reported only once for each function it appears in

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