regulator: 88pm800: Add missing n_voltages setting for bucks

Some regulator APIs are broken if n_voltages setting is not set.
e.g. regulator_count_voltages(), regulator_list_voltage(), and
regulator_can_change_voltage() do not work if n_voltages setting is missing.

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