phy: rcar-gen3-usb2: check dr_mode for otg mode

The previous code assumed a channel has otg capability if a channel
has interrupt property. But, it is not good because:
 - Battery charging feature also needs interrupt property.
 - Some R-Car Gen3 SoCs (e.g. R-Car D3) don't have OTG capability.

So, this patch checks whether usb 2.0 host node has dr_mode property or
not. If it has 'dr_mode = "otg";', this driver enables otg capability.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>