MIPS: jz4740: Remove custom GPIO code

All the drivers for the various hardware elements of the jz4740 SoC have
been modified to use the pinctrl framework for their pin configuration
needs.
As such, this platform code is now unused and can be deleted.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>