gpio/ep93xx: Add missing EP93XX_GPIO_PHYS_BASE define

The patch to convert of the ep93xx gpio driver to a platform_driver is
missing the define for the memory resource that is passed to the driver.

This is needed to prevent a build error.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>