phy: rcar-gen3-usb2: change the mode to OTG on the combined channel

To use the channel 0 of R-Car gen3 as periperal mode, This patch changes
the mode to OTG instead of HOST. Then, this driver needs to set some
registers to enable host mode and detects ID pin and VBUS pin at
phy_init() timing.

For now, the channel 0 can be used as host mode only.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>