drm: of: Add drm_of_get_data_lanes_count and drm_of_get_data_lanes_ep

Add helper function to count and sanitize DT "data-lanes" property
and return either error or the data-lanes count. This is useful for
both DSI and (e)DP "data-lanes" property. The later version of the
function is an extra wrapper which handles the endpoint look up by
regs, that's what majority of the drivers duplicate too, but not all
of them.

Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
To: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220524010522.528569-1-marex@denx.de