MAINTAINERS: add Sam Ravnborg for drm/atmel_hlcdc

I have agreed with Boris Brezillon that we will share the
maintainer role for the drm/atmel_hlcdc driver.
Nicolas Ferre from Microchip has donated a few boards that
allows me to test things - thanks!

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Boris Brezillon <bbrezillon@kernel.org>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Claudiu Beznea <claudiu.beznea@microchip.com>
Cc: Peter Rosin <peda@axentia.se>
Link: https://patchwork.freedesktop.org/patch/msgid/20190627211643.GA19853@ravnborg.org