phy: Add binding document for Pistachio USB2.0 PHY

Add a binding document for the USB2.0 PHY found on the IMG Pistachio SoC.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: devicetree@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: James Hartley <james.hartley@imgtec.com>
Cc: Damien Horsley <Damien.Horsley@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/9727/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>