mfd: Remove gpio support from tps6586x core driver

The GPIO functionality of device tps6586x is added through
platform gpio driver and it can be register as the mfd sub
device and hence removing the duplicates code which register
the gpio functionality from core driver.

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