drm/panel: simple: Add Powertip PH800480T013 panel

Add support for Powertip PH800480T013 800x480 parallel LCD, this
one is used in the Raspberry Pi 7" touchscreen display unit.

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/20200728121246.23304-3-marex@denx.de