gpio/ep93xx: move driver to drivers/gpio

The GPIO driver should reside in drivers/gpio.

v3: Change Kconfig option to def_bool y
v2: Make the Kconfig symbol a silent option, dependent on ARCH_EP93XX

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