gpio: tps6586x: Add gpio support through platform driver

Converting the gpio driver of tps6586x to a platform
driver in place of registering the gpio through core
driver.
The motivation of the change is:
- This is inline with the mfd drivers implementation.
- This will move the related gpio support to gpio driver
  folder where all gpio related drivers are available.
  This will be easy the maintenance and enhancement is
  anything done for gpio.
- The gpio functionality can be selected through config
  variable.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>