ARM: dts: dra7x-evm: beagle-x15: Fix USB Host

In commit 87517d26 ("ARM: dts: dra7-evm: Add extcon nodes for USB")
we enabled Extcon USB gpio to tackle the USB ID pin and get
peripheral mode to work.

But the extcon-gpio-usb driver [1] didn't make it into v4.0
and this makes the USB driver defer probe indefinitely breaking
USB Host functionality.

As a temporary fix we remove the extcon handle from the
USB controller and add it back when the extcon driver
merges in v4.1.

[1] - https://lkml.org/lkml/2015/2/2/187



Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>