regulator: tps6524x: Convert to regulator_list_voltage_table()

This patch adds fixed_5000000_voltage table for fixed voltage,
so we can convert regulator_ops to regulator_list_voltage_table.

We can differentiate fixed voltage by checking rdev->desc->n_voltages == 1,
thus remove the FIXED_VOLTAGE flag usage.

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