Documentation: devicetree: dwc2: Deprecate g-tx-fifo-size

This property is not needed because the periodic fifos are not
configurable. So it was incorrect to add this property in the first
place.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>