dt-bindings: mfd: ti,am3359-tscadc: Add missing power-domains property

Add optional power-domains property to avoid the following dt-schema
failures:

tscadc@40200000: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Matt Ranostay <mranostay@ti.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221025080014.403457-1-mranostay@ti.com