drm/msm/dsi: Add byte_intf_clk

DSI6G v2.0+ blocks have a new clock input to them called
byte_intf_clk. It's rate is to be set as byte_clk / 2.

Within the clock controller (CC) subsystem, this clock is a
child/descendant of the byte_clk.

Set it up as an optional clock in the DSI host driver. Make sure
that we enable/set its rate only after we configure byte_clk.
This is required for the ancestor clocks in the CC to be
configured correctly.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>