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>

arch/arm/Kconfig

@@ -645,6 +645,7 @@ config ARCH_SHMOBILE_LEGACY
 
 config ARCH_RPC
 	bool "RiscPC"
+	depends on MMU
 	select ARCH_ACORN
 	select ARCH_MAY_HAVE_PC_FDC
 	select ARCH_SPARSEMEM_ENABLE