msm: clock-copper: Allow the bus driver to control mmssnoc_axi_clk

Previously, the bus driver would set the rate on axi_clk_src
directly. However, enabling/disabling this clock has no effect
since the clock driver does not touch the root enable bit.
Allow the bus driver to control the mmssnoc_axi_clk, which
is a branch off of axi_clk_src, as has been done for the
OCMEM NOC clock.

Change-Id: I4f0619354a2c1525e885471d7bfde8206ca1d9f6
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>