Add Makefile item for soc specific driver for rk3399 mipi dsi.

Signed-off-by: Eric Gao <eric.gao@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>