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>