move twl4030-gpio to drivers/gpio

move twl4030-gpio to drivers/gpio

Move the twl4030 GPIO support from drivers/i2c/chips to drivers/gpio,
which is a more appropriate home for this code.

The Kconfig symbol name is changed to match the GPIO_* convention for
such symbols, so config files must change in the obvious ways (Kconfig
will prompt you).  There's now some helptext.

It can now be compiled as a module, should anyone really want to
do that; that'll be mostly useful for test builds.  Sanity check the
IRQ range we're given.

Initialization order needed a bit of work too:  core add_children()
called only after IRQs get set up, gpio uses subsys_initcall.  This
depends on a patch making i2c driver model init use postcore_initcall.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>