videomode: implement public of_get_display_timing()

The current of_get_display_timings() reads multiple display timings,
allocating memory for the entries. However, most of the time when
parsing display timings from DT data is needed, there's only one display
timing as it's not common for a LCD panel to support multiple videomodes.

This patch creates a new function:

int of_get_display_timing(struct device_node *np, const char *name,
               struct display_timing *dt);

which can be used to parse a single display timing entry from the given
node name.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>