msm: clock-local2: Add branch_clk_set_flags() for memory retention control

Some branch clocks have a configuration bit associated with them
that determines whether or not SRAM memories in their clock domain
will lose or retain state when the clock is halted.

By default (out of reset) these bits are set so that, even when the
clocks are halted, memory state is retained. This is useful for
drivers that wish to take advantage of temporary clock gating as an
inexpensive way to save power, without needing to re-initialize the
hardware they control again after a clock is re-enabled.

On the other hand, some additional power can be saved by clearing
these bits and allowing the core's memory to collapse. To take
advantage of this, add a means of setting this bit through the
clk_set_flags() API.

Change-Id: I71b68e18d166abce77ff2a2da81f1dff76467d17
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>