ARM: sa11x0: assabet: fix build warning

Since a32618d2 (ARM: pgtable: switch to use pgtable-nopud.h), assabet
warns as follows:

arch/arm/mach-sa1100/assabet.c: In function 'map_sa1100_gpio_regs':
arch/arm/mach-sa1100/assabet.c:264: warning: passing argument 1 of 'pmd_offset' from incompatible pointer type

Fix this by adding the necessary pud_offset() macro.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>