drm/panel: Add driver for Sony ACX424AKP panel

The Sony ACX424AKP is a command/videomode DSI panel for
mobile devices. It is used on the ST-Ericsson HREF520
reference design. We support video mode by default, but
it is possible to switch the panel into command mode
by using the bool property "dsi-command-mode".

Cc: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200109072815.334867-1-linus.walleij@linaro.org