gpio: split GPIO drivers in submenus

Create Kconfig submenus for memory mapped, I2C, MFD,
PCI, SPI and USB GPIO drivers to help navigate the forest
of drivers in this subsystem. The I2C, SPI and USB menus
get dependencies so we don't have to see them unless we
have the required subsystem enabled in the first place.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>