usb: ehci-mx6: use phy_type from device tree

Allow using different PHY interfaces for multiple USB controllers. When no
value is set in DT, we fall back to CONFIG_MXC_USB_PORTSC for now to stay
compatible with current board configurations.

This also adds support for the HSIC mode of the i.MX7.

Signed-off-by: Markus Niebel <Markus.Niebel@ew.tq-group.com>
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>