MLK-18878 DTS: imx8mq: Change USB nodes compatible strings

MLK-18878 DTS: imx8mq: Change USB nodes compatible strings

We enabled the DM for xhci-imx8m driver which uses compatible string
"fsl, imx8mq-dwc3". But found a issue that u-boot will get four USB bus not two.
The reason is the low level driver xhci-dwc3 also adds the DM support which uses
compatible string "snps,dwc3". Thus, one USB node and its dwc3 subnode are both
binded as independent USB bus.

Since the xhci-imx8m driver uses xhci-dwc3 as low level driver, to fix the issue
we add -u-boot.dtsi files to change the USB node compatible string to
"simple-bus" and change dwc3 node compatibe string to "fsl, imx8mq-dwc3". Then
xhci-dwc3 DM driver won't bind any node.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 7542babca0c1303d469642c346cc927de3facf12)