drm/panel: simple: Add support for Giantplus GPG482739QS5

This patch adds support for the GiantPlus GPG48273QS5 4.3" WQVGA TFT LCD
panel to the simple-panel driver.

This panel is connected via a parallel bus and uses both HSYNC and
VSYNC, whose lengths are unfortunately not clearly defined. The
datasheet only specifies the front- and backporch length, but the timing
diagram suggests that both sync signals should be asserted for exactly
one clock cycle.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>