gpio: move MFD GPIO drivers under their own comment

Get rid of AC97, MODULbus and other weird subheadings for
GPIO drivers. Move all MFD drivers out of I2C etc and in under
the MFD comment. This is too weird as it is and makes no
sense, if the dependent parent driver is MFD, group these as
MFD GPIO drivers. Alphabetize and move this comment group
inbetween "I2C" and "PCI" to also have the groups in
alphabetic order.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>