drm/bridge/synopsys: dsi: add dual-dsi support

Allow to also drive a slave dw-mipi-dsi controller in a dual-dsi
setup. This will require additional implementation-specific
code to look up the slave instance and do specific setup.
Also will probably need code in the specific crtcs as dual-dsi
does not equal two separate dsi outputs.

To activate, the implementation-specific code should set the slave
using dw_mipi_dsi_set_slave() before calling __dw_mipi_dsi_bind().

v2:
- expect real interface number of lanes
- keep links to both master and slave
v3:
- remove unneeded separate variables
- remove unneeded second slave settings
- disable slave before master
- lane-sum calculation comments

Signed-off-by: Nickey Yang <nickey.yang@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181001123845.11818-7-heiko@sntech.de