drm: Add bus flag for Sharp-specific signals

Add the DRM_BUS_FLAG_SHARP_SIGNALS to the drm_bus_flags enum.

This flags can be used when the display must be driven with the
Sharp-specific signals SPL, CLS, REV, PS.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190603153120.23947-2-paul@crapouillou.net