arm64: dts: verdin-imx8mm: add otg2 pd to usbphy

The Verdin iMX8M Mini System on Module does not have USB-ID signal
connected on Verdin USB_2 (usbotg2). On Verdin Development board this is
no problem, as we have connected a USB-Hub that is always connected.

However, if Verdin USB_2 is desired to be used as a single USB-Host port
the chipidea driver does not detect if a USB device is plugged into this
port, due to runtime pm shutting down the PHY.

Add the power-domain &pgc_otg2 to &usbphynop2 in order to detect
plugging events and enumerate the usb device.

Fixes: 6a57f224 ("arm64: dts: freescale: add initial support for verdin imx8m mini")
Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>