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 <w.sang@pengutronix.de>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
diff --git a/arch/powerpc/platforms/86xx/Kconfig b/arch/powerpc/platforms/86xx/Kconfig
index a0b5638..8d6599d 100644
--- a/arch/powerpc/platforms/86xx/Kconfig
+++ b/arch/powerpc/platforms/86xx/Kconfig
@@ -4,6 +4,7 @@
 	depends on 6xx
 	select FSL_SOC
 	select ALTIVEC
+	select ARCH_WANT_OPTIONAL_GPIOLIB
 	help
 	  The Freescale E600 SoCs have 74xx cores.