regulator: mt6358: Make MT6366 vcn18 LDO configurable

regulator: mt6358: Make MT6366 vcn18 LDO configurable

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@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230928085537.3246669-10-wenst@chromium.org
Signed-off-by: Mark Brown <broonie@kernel.org>