ARM: mmu: Call debug_ll_io_init if no map_io function is specified

More and more sub-architectures are using only the debug_ll_io_init
function as the map_io function. Make the core code call this function
if no function is specified in the machine description to remove some
boilerplate code.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>