usb: misc: usb3503: Don't specify all DT properties as required

The binding document says that all properties are required but in fact
almost all are optional (and should be) - update the document to reflect
this.

Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>