display: ssd1673: add support for ssd16xx monochrome controllers

From the driver point of view, monochrome controllers from the ssd16xx
family mostly differ by the amount of row and columns that are
supported. If they support more than 256 rows and/or columns the
corresponding size or position is sent using 2 bytes instead of 1 byte.

This patch therefore adds the width-bits and height-bits DT properties
to make this configurable.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>