dt-bindings: phy: Remove phy-stih41x-usb binding

The driver was removed in 2016 in commit fb954c48 ("phy:
stih41x-usb: Remove usb phy driver and dt binding  documentation.") and
somehow the DT binding got dropped despite the subject.

Cc: Peter Griffin <peter.griffin@linaro.org>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Link: https://lore.kernel.org/r/20200826184851.3431531-2-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/phy/phy-stih41x-usb.txt

-STMicroelectronics STiH41x USB PHY binding
-------------------------------------------
-
-This file contains documentation for the usb phy found in STiH415/6 SoCs from
-STMicroelectronics.
-
-Required properties:
-- compatible	: should be "st,stih416-usb-phy" or "st,stih415-usb-phy"
-- st,syscfg	: should be a phandle of the syscfg node
-- clock-names	: must contain "osc_phy"
-- clocks	: must contain an entry for each name in clock-names.
-See: Documentation/devicetree/bindings/clock/clock-bindings.txt
-- #phy-cells	: must be 0 for this phy
-See: Documentation/devicetree/bindings/phy/phy-bindings.txt
-
-Example:
-
-usb2_phy: usb2phy@0 {
-	compatible	= "st,stih416-usb-phy";
-	#phy-cells	= <0>;
-	st,syscfg	= <&syscfg_rear>;
-	clocks		= <&clk_sysin>;
-	clock-names	= "osc_phy";
-};