clk: imx8mp: use usb_core_ref for usb_root_clk

Upstream commit 7a2c3be9 ("clk: imx8mp: Fill in DWC3 USB, USB PHY,
HSIOMIX clock") added usb_core_ref for USB Controller but never set it
to be used as a clock source, using rather "osc_32k" instead.

This produces following boot log message:
"clk_register: failed to get osc_32k device (parent of usb_root_clk)"

Fix the USB controller clock source by using usb_core_ref instead of
osc_32k.

Fixes: 7a2c3be9 ("clk: imx8mp: Fill in DWC3 USB, USB PHY, HSIOMIX clock")
Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>