pmic:max8997: Function for calculating LDO internal register value

Function for calculating LDO internal register value from passed micro
Volt.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Stefano Babic <sbabic@denx.de>