arm: dts: k3-j721e-r5-*.dts: Fix clock-names property in the usb0 instance

In the cdns3 usb driver, the clock name looked for is ref. Therefore, fix
the clock-names property in usb0 instance for proper initialization of
cdns3 usb gadget driver.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>