Documentation: samsung-phy: Add exynos-pcie-phy binding

Add the exynos-pcie-phy binding for Exynos PCIe PHY.  This is for using
generic PHY framework.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>