regulator: twl: Fix list_voltate for twl6030ldo_ops

According to the datasheet, the voltage for twl6030ldo_ops is not linear for
all cases. Linear mapping is only for the selection code from
00000001 to 00011000.

Table 9. LDO Output Voltage Selection Code
CODE     VOUT(V)    COD      VOUT(V)    CODE     VOUT(V)    CODE     VOUT(V)
00000000 0          00001000 1.7        00010000 2.5        00011000 3.3
00000001 1.0        00001001 1.8        00010001 2.6        00011001 Reserved
00000010 1.1        00001010 1.9        00010010 2.7        00011010 Reserved
00000011 1.2        00001011 2.0        00010011 2.8        00011011 Reserved
00000100 1.3        00001100 2.1        00010100 2.9        00011100 Reserved
00000101 1.4        00001101 2.2        00010101 3.0        00011101 Reserved
00000110 1.5        00001110 2.3        00010110 3.1        00011110 Reserved
00000111 1.6        00001111 2.4        00010111 3.2        00011111 2.75

This patch implements the list_voltage callback based on above table.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Tested-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>