Davinci: gpio - structs and functions renamed

Renamed gpio types to something more sensible:
	struct gpio_controller	--> struct davinci_gpio_regs
	struct davinci_gpio	--> struct davinci_gpio_controller
	gpio2controller()	--> gpio2regs()
	irq2controller()	--> irq2regs()

This change also moves davinci_gpio_controller definition to gpio.h.
Eventually, the gpio registers structure will be moved to gpio.c and no longer
a common cross-soc definition.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Tested-by: Sandeep Paulraj <s-paulraj@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>