regulator: Add bindings for Richtek RT5190A PMIC

Add bindings for Richtek RT5190A PMIC.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/1646812903-32496-2-git-send-email-u0084500@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>