clk: imx7d: Add USB clock information

Add USB clock information, the pll_usb_main_clk is USB_PLL at CCM
which is the output of USBOTG2 PHY.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Irina Tirdea <irina.tirdea@nxp.com>
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>