omap: gpio: Use generic API

Convert all OMAP specific functions to use the common API
definitions in include/asm/gpio.h. In the process, made
few additional changes:
 - Use -EINVAL consistently. -1 was used in many places.
 - Removed one-liner static functions that were used only
   once. Replaced the content as necessary.
 - Combines implementation of functions omap_get_gpio_dataout()
   and omap_get_gpio_datain(). To do so, new static function
   _get_gpio_direction() was added.

Signed-off-by: Sanjeev Premi <premi@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>