phy: rockchip-inno-usb2: support to force otg mode

phy: rockchip-inno-usb2: support to force otg mode

This patch creates an usb2 phy attribute group and
provides an attribute "otg_mode" for otg port to
force otg mode independently of the voltage of otg
id pin.

In order to implement the force mode function, we can
select otg plug indicator output (AKA iddig) from GRF,
and set GRF USB otg plug indicator to "0" or "1" to
control iddig status.

We only support rk322x/rk3328 to force otg mode for
the time being.

And we need to disable usb auto suspend function if
we want to force otg mode. Add 'usbcore.autosuspend=-1'
in cmdline to disable usb auto suspend.

Usage:
[1] Force host mode
    echo host > /sys/devices/platform/<u2phy dev name>/mode

[2] Force peripheral mode
    echo peripheral > /sys/devices/platform/<u2phy dev name>/mode

[3] Force otg mode
    echo otg > /sys/devices/platform/<u2phy dev name>/mode

Legacy Usage:
[1] Force host mode
    echo 1 > /sys/devices/platform/<u2phy dev name>/mode

[2] Force peripheral mode
    echo 2 > /sys/devices/platform/<u2phy dev name>/mode

[3] Force otg mode
    echo 0 > /sys/devices/platform/<u2phy dev name>/mode

Change-Id: I875b60b0390e3bd9af34b740cba8f5d53e1df752
Signed-off-by: William Wu <william.wu@rock-chips.com>