phy: freescale: Break dependency on SOC_IMX8MQ for USB PHY

Since this is going to be used on more SoCs than just i.MX8MQ,
make the dependency here more generic.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>