phy: phy-rockchip-inno-usb2: drop reading the utmi-avalid property

That property is no used in mainline and is not documented. The only
board using that property is the rk33-99-evb-rev1 and -rev2 in the
vendor kernel. The existence of a further -rev3 (which also looks way
better cared for compared rev1+2) indicates that the older ones are
probably some sort of preproduction models, where some wiring (on the soc
or board) may have gone wrong.

It is also not clear why this is a hardware-description or a DT
property, so, as noboby seems to care of this just drop reading that
property.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>