dt-bindings: phy: uniphier-pcie: Add bindings for NX1 SoC

Update PCIe-PHY binding document for UniPhier NX1 SoC. Add a compatible
string for the SoC to the document.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1635503947-18250-4-git-send-email-hayashi.kunihiko@socionext.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>