regulator: lp8788: Implement list_voltage for lp8788_ldo_voltage_fixed_ops

For fixed voltage, we can just set min_uV and use regulator_list_voltage_linear
for list_voltage callback. Regulator core will call list_voltage(rdev, 0) if
both get_voltage get_voltage_sel are not implemented. Thus we can also remove
lp8788_ldo_fixed_get_voltage() function.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>