drm: panel: Add Himax HX8394 panel controller driver

The driver is for panels based on the Himax HX8394 controller, such as the
HannStar HSD060BHW4 720x1440 TFT LCD panel that uses a MIPI-DSI interface.

Signed-off-by: Kamil Trzciński <ayufan@ayufan.eu>
Co-developed-by: Ondrej Jirman <megi@xff.cz>
Signed-off-by: Ondrej Jirman <megi@xff.cz>
Co-developed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Tom Fitzhenry <tom@tom-fitzhenry.me.uk>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102230733.3506624-3-javierm@redhat.com