ARM: make arch_ret_to_user macro optional

Only 3 platforms need arch_ret_to_user macro, so add ARCH_HAS_RET_TO_USER
kconfig option and make iop13xx, iop32x and iop33x select it.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Nicolas Pitre <nico@linaro.org>