dt-bindings: Add vendor prefix for Powertip

The Powertip Tech. Corp. is an LCD panel manufacturer.

Signed-off-by: Marek Vasut <marex@denx.de>
To: dri-devel@lists.freedesktop.org
Cc: Eric Anholt <eric@anholt.net>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200728121246.23304-1-marex@denx.de