[ARM] Orion: rework MPP handling

Instead of having board code poke directly into the MPP configuration
registers, and separately calling orion5x_gpio_set_valid_pins() to
indicate which MPP pins can be used as GPIO pins, introduce a helper
function for configuring the roles of each of the MPP pins, and have
that helper function handle gpio validity internally.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Sylver Bruneau <sylver.bruneau@googlemail.com>
Acked-by: Russell King <linux@arm.linux.org.uk>