drm: atmel-hlcdc: add config option for clock selection

SAM9x60 LCD Controller has no option to select clock source as previous
controllers have. To be able to use the same driver even for this LCD
controller add a config option to know if controller supports this.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1556195748-11106-2-git-send-email-claudiu.beznea@microchip.com