OMAP: GPIO: prepare for platform driver

Prepare for implementing GPIO as a platform driver.

Modifies omap_gpio_init() to make use of omap_gpio_chip_init()
and omap_gpio_mod_init(). omap_gpio_mod_init() does the module init
by clearing the status register and initializing the GPIO control register.
omap_gpio_chip_init() initializes the chip request, free, get, set and
other function pointers and sets the gpio irq handler.

This is only to reorganize the code so that the "omap gpio platform driver
implementation patch" looks cleaner and better to review.

Signed-off-by: Charulatha V <charu@ti.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>