ARM: fix misplaced arch_virt_to_idmap()

Olof Johansson reported:

In file included from arch/arm/include/asm/page.h:163:0,
                 from include/linux/mm_types.h:16,
                 from include/linux/sched.h:24,
                 from arch/arm/kernel/asm-offsets.c:13:
arch/arm/include/asm/memory.h: In function '__virt_to_idmap':
arch/arm/include/asm/memory.h:300:6: error: 'arch_virt_to_idmap' undeclared (first use in this function)

caused by arch_virt_to_idmap being placed inside a different
preprocessor conditional to its user.  Move it along side its user.

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