ARM: fix __virt_to_idmap build error on !MMU

Fengguang Wu reports that building ARM with !MMU results in the
following build error:

   arch/arm/kernel/built-in.o: In function `__soft_restart':
>> :(.text+0x1624): undefined reference to `arch_virt_to_idmap'

Fix this by adding an appropriate IS_ENABLED(CONFIG_MMU) into the
__virt_to_idmap() inline function.

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