regulator: max8998 BUCK1/2 internal voltages and indexes defined

BUCK1/2 internal voltages and indexes defined in the struct max8998_data
max_get_voltage_register now uses index values to chose proper register
More generic BUCK1/2 registers names provided

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>