MIPS: ath79: Fix the USB PHY reset names

The binding for the USB PHY went thru before the driver. However the
new version of the driver now use the PHY core support for reset, and
this expect the reset to be named "phy". So remove the "usb-" prefix
from the the reset names.

Signed-off-by: Alban Bedel <albeu@free.fr>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/15282/
Cc: linux-kernel@vger.kernel.org
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Antony Pavlov <antonynpavlov@gmail.com>
Cc: devicetree@vger.kernel.org
Cc: linux-mips@linux-mips.org