drm/msm/dsi: Add configuration for 8x76

MSM8976, MSM8976 and APQ variants have DSI version 3:10040002
(DSI 6G V1.4.2), featuring two DSIs.
They need three clocks (mdp_core, iface, bus), one GDSC and
two vregs, VDDA at 1.2V and VDDIO at 1.8V.

Signed-off-by: AngeloGioacchino Del Regno <kholk11@gmail.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>