The VCN18 regulator on the MT6366 (only) actually has a wide
configurable range of voltages, even though its name suggests a fixed
output voltage.
Convert it from a fixed LDO to a configurable LDO. Its range of settings
is the same as the VM18 regulator, which is missing and will be added in
a subsequent patch.
Signed-off-by: Chen-Yu Tsai <wenst@xxxxxxxxxxxx>