arm64: dts: imx8: fix USB 3.0 Gadget Failure in QM & QXPB0 at super speed

Resolve USB 3.0 gadget failure for QM and QXPB0 in super speed mode with
single IN and OUT endpoints, like mass storage devices, due to incorrect
ACTUAL_MEM_SIZE in ep_cap2 (32k instead of actual 18k). Implement dt
property cdns,on-chip-buff-size to override ep_cap2 and set it to 18k for
imx8QM and imx8QXP chips. No adverse effects for 8QXP C0.

Cc: stable@vger.kernel.org
Fixes: dce49449

 ("usb: cdns3: allocate TX FIFO size according to composite EP number")
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>