gpio: move mpc8xxx/512x gpio driver to drivers/gpio

gpio: move mpc8xxx/512x gpio driver to drivers/gpio

Move the driver to the place where it is expected to be nowadays. Also
rename its CONFIG-name to match the rest and adapt the defconfigs.
Finally, move selection of REQUIRE_GPIOLIB or WANTS_OPTIONAL_GPIOLIB to
the platforms, because this option is per-platform and not per-driver.

Signed-off-by: Wolfram Sang <[email protected]>
Cc: Anatolij Gustschin <[email protected]>
Cc: Grant Likely <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Acked-by: Grant Likely <[email protected]>
Signed-off-by: Anatolij Gustschin <[email protected]>