dt-bindings: PCI: rcar-pci-host: Add optional regulators

Support regulators found on the KingFisher board for miniPCIe (1.5 and
3.3v). For completeness, describe a 12v regulator while we are here.

Link: https://lore.kernel.org/linux-pci/20231105092908.3792-2-wsa+renesas@sang-engineering.com
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>