ARM: dove: fix missing __init section of dove_mpp_gpio_mode

Legacy dove_mpp_gpio_mode calls orion_gpio_set_valid which is in
__init section. Offending function is not, so this causes a section
mismatch. To fix the mismatch, also move dove_mpp_gpio_mode to
__init section.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>