regulator: tps80031: Fix LDO2 track mode for TPS80031 or TPS80032-ES1.0

Currently we have a special code in tps80031_ldo_set_voltage_sel() to handle
LDO2 track mode for TPS80031 or TPS80032-ES1.0. The purpose is to address below
issues:

Issue description:
- LDO2 traking mode is enabled
- LDO2 tracks SMPS2 voltage.
- LDO2 automatically switch-off when LDO2_CFG_VOLTAGE is changed to some discrete values (non exhaustive list):
            00011001, 00011010, 00011011, 00011100, .
- LDO2 switch-on again when LDO2_CFG_VOLTAGE is changed to other values (non exhaustive list):
        00011000, 00010111, .

LDOs have reserved codes. For these codes, LDO is switch-off.

In tracking, LDO2 ref comes from SMPS2.
However LDO2 enable is still gated by LDO2 VSEL decoding.
As a result, in tracking mode LDO2 will be disabled for following code (SMPS VSEL format):
000000 & 100000 (MSB not decoded)
011001 & 111001 (MSB not decoded)
011010 & 111010 (MSB not decoded)
011100 & 111100 (MSB not decoded)
011101 & 111101 (MSB not decoded)
011110 & 111110 (MSB not decoded)

However, current code has below bugs:
1. It uses regulator_list_voltage_linear, so list_voltage() still shows above
   invalid selectors have supported voltage.
2. Current code may return -EINVAL in tps80031_ldo_set_voltage_sel() for
   supported voltage. This is because when we use regulator_list_voltage_linear
   as list_voltage callback, regulator core will default use
   regulator_map_voltage_linear(). regulator_map_voltage_linear() has an
   assumption that the voltages are linear which is not true for this case.

   For example,
   when request voltage range is: min_uV=950000 uV && max_uV=1200000 uV
   regulator_map_voltage_linear() returns the selector is 29 (0x1D),
   set_voltage_sel() returns -EINVAL.
   (The selector is in invalid range, 0x19 ~ 0x1f).

   In above case, map_voltage() should find the lowest valid voltage within
   specific range (selector = 0x20) and set_voltage_sel() should successfully
   set the voltage to 987500 uV.

This patch fixes these issues by:
1. Add checking valid setting for LDO2 track mode of TPS80031 or TPS80032-ES1.0
   in list_voltage.  So it returns -EINVAL for invalid selectors.
2. Implement tps80031_ldo_map_voltage, use regulator_map_voltage_iterate()
   to find the lowest voltage within specific range for TPS80031 or
   TPS80032-ES1.0. This is required when the voltage map is no longer linear.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>