regulator: max8998 BUCK1/2 voltage change with use of GPIOs

max8998_pmic_probe:
- modified to check if valid pins are defined at platform
data
- maximal voltage values (predefined at platform data) are uploaded to
max8998 device

max8998_set_voltage_buck:
- BUCK1/2 voltages change between values already defined
- Checks if valid GPIO pins are passed from platform data
- If requested voltage cannot be satisfied from already defined values,
then one of free slots is used
- Predefined maximum voltages (as defined at platform data) are always
available

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>