ep93xx: Configure GPIO ports in core code

Move the pinmux setting of the EP93xx GPIOs to the core code. This
removes the need for the GPIO driver to have access to the system
controller registers.

Signed-off-by: Ryan Mallon <rmallon@gmail.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Reviewed-by: Mika Westerberg <mika.westerberg@iki.fi>
Acked-by: Hartley Sweeten <hsweeten@visionengravers.com>