usb: dwc3: imx8mp: detect dwc3 core node via compatible string

New schema of usb controller DT-node should be named with prefix
"^usb(@.*)?", dt changed the node name, but missed counter part
change in driver, fix it by switching to use compatible string as
the dwc3 core compatible string keeps "snps,dwc3" in all dt.

Fixes: d1689cd3

 ("arm64: dts: imx8mp: Use the correct name for child node "snps, dwc3"")
Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Li Jun <jun.li@nxp.com>
Link: https://lore.kernel.org/r/1619765836-20387-1-git-send-email-jun.li@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>