dt: bindings: dwc2: fix required value for the phy-names property

"74084844 usb: gadget: s3c-hsotg: enable generic phy support" introduces
generic phy support to the dwc2.txt binding and the s3c-hsotg driver
which implements support for the binding. The binding documentation
incorrectly states that the phy-names property will be "device". The
binding example, driver, and one dts user all implement the phy-names
property as requiring "usb2-phy".

Fix the dwc2.txt binding documentation to correctly specify "usb2-phy"
as the appropriate value for phy-names.

Reported-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>