phy: Be able to get phy from PHY provider

phy: Be able to get phy from PHY provider

In case of phy are provided from a PHY provider nodes as following:

usbphyc: usb-phy@5a006000 {
	compatible = "st,stm32mp1-usbphyc";
	reg = <0x5a006000 0x1000>;
	clocks = <&rcc_clk USBPHY_K>;
	resets = <&rcc_rst USBPHY_R>;
	#address-cells = <1>;
	#size-cells = <0>;

	usbphyc_port0: usb-phy@0 {
		reg = <0>;
		phy-supply = <&vdd_usb>;
		vdda1v1-supply = <&reg11>;
		vdda1v8-supply = <&reg18>
		#phy-cells = <0>;
	};

	usbphyc_port1: usb-phy@1 {
		reg = <1>;
		phy-supply = <&vdd_usb>;
		vdda1v1-supply = <&reg11>;
		vdda1v8-supply = <&reg18>
		#phy-cells = <1>;
	};
};

and PHY are called as following:

usbh_ehci: usbh-ehci@5800d000 {
	compatible = "generic-ehci";
	reg = <0x5800d000 0x1000>;
	clocks = <&rcc_clk USBH>;
	resets = <&rcc_rst USBH_R>;
	interrupts = <GIC_SPI 75 IRQ_TYPE_NONE>;
	companion = <&usbh_ohci>;
	phys = <&usbphyc_port0>;
	phy-names = "usb";
	status = "okay";
};

generic_phy_get_by_index() must be updated to first look for
PHY phandle as previously and in case of error looks for PHY
provider by finding the parent's current node which is the PHY
provider.
args (ofnode_phandle_args struct) must also be updated by inserting
the phy index into the PHY provider as args[0].

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>