In current code, .list_voltage and .set_voltage_sel callbacks for BUCK7 and
BUCK8 return -EINVAL.

This patch adds s5m8767_buck78_ops for BUCK7 and BUCK8 which does not set
.list_voltage, .get_voltage_sel and .set_voltage_sel. ( This has the same
effect of returning -EINVAL in the callbacks)

Then for all the users of s5m8767_list_voltage, we don't need to worry about
the case reg_voltage_map[reg_id] is NULL.
So we can convert s5m8767_list_voltage to regulator_list_voltage_linear.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>