Documentation: dt-bindings: rename meson-usb2-phy to meson8b-usb2-phy

The corresponding driver only supports the USB PHY on Meson8b and GXBB
SoCs. Newer SoC versions are using a different USB PHY implementation,
which will mean that a new driver is required. Thus make sure that our
naming is specific enough so it does not conflict with upcoming drivers.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>