mfd: Fix tps6586x GPIO value setting

In the current code, every time we set a TPS6586x GPIO pin,
the tps6586x_gpio_set function is resetting all other GPIO pins.
We need to update the right GPIOxOUT bit of the GPIOSET2 register instead
of overriding the full value.

Tested by setting sequentially GPIO2 and GPIO3 and verifying the
pins voltage.

Change-Id: I560edde146c5425cce37432c4ee91569eea5adcf
Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>