drm/panel-simple: Add 50 Hz mode to the Frida FRD350H54004 panel

By changing the pixel clock and the length of the back porch, it is
possible to obtain a perfect 50 Hz refresh rate.

v2: Rebase on drm-misc-next

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716125647.10964-2-paul@crapouillou.net