dt-bindings: ata: Add UniPhier controller binding

Add UniPhier SATA controller compatible string to the platform binding.
This controller needs three reset controls for Pro4 SoC, or two reset
controls for PXs2 and PXs3 SoCs.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>