ARM: make RiscPC depend on MMU

RiscPC fails to build if MMU is disabled:

arch/arm/mach-rpc/ecard.c: In function 'ecard_init_pgtables':
arch/arm/mach-rpc/ecard.c:229:2: error: implicit declaration of function 'pgd_offset' [-Werror=implicit-function-declaration]

arrange for RiscPC to depend on MMU.

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