staging: hikey9xx: add USB physical layer for Kirin 3670

Add the Hisilicon Kirin 3670 USB phy driver.

This driver was imported from Linaro's official Hikey 970
tree, from the original patch, removing the addition of
the dwg3-specific parts, and getting the missing SoB from
its original author:

	https://github.com/96boards-hikey/linux/commit/9d168f580c9977f9c7f48b228b72035e2f6e3eba#diff-93bb70bc97bdd7be752cb6722adf2124

[mchehab: moved to staging and dropped Makefile/Kconfig changes]
Signed-off-by: Yu Chen <chenyu56@huawei.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/0e6b2fa68cabd317511637fdfdeadc574196ea90.1600338981.git.mchehab+huawei@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/hikey9xx/phy-hi3670-usb3.txt

+Hisilicon Kirin970 usb PHY
+-----------------------
+
+Required properties:
+- compatible: should be "hisilicon,kirin970-usb-phy"
+- #phy-cells: must be 0
+- hisilicon,pericrg-syscon: phandle of syscon used to control phy.
+- hisilicon,pctrl-syscon: phandle of syscon used to control phy.
+- hisilicon,sctrl-syscon: phandle of syscon used to control phy.
+- hisilicon,usb31-misc-syscon: phandle of syscon used to control phy.
+- eye-diagram-param: parameter set for phy
+- usb3-phy-tx-vboost-lvl: parameter set for phy
+Refer to phy/phy-bindings.txt for the generic PHY binding properties
+
+Example:
+	usb_phy: usbphy {
+		compatible = "hisilicon,kirin970-usb-phy";
+		#phy-cells = <0>;
+		hisilicon,pericrg-syscon = <&crg_ctrl>;
+		hisilicon,pctrl-syscon = <&pctrl>;
+		hisilicon,sctrl-syscon = <&sctrl>;
+		hisilicon,usb31-misc-syscon = <&usb31_misc>;
+		eye-diagram-param = <0xFDFEE4>;
+		usb3-phy-tx-vboost-lvl = <0x5>;
+	};