msm: msm_bus: Log Bus client request transitions

Use kernel's ftrace support to capture bus client request
transitions which are useful for debugging issues related
to power consumption.
Usage:

mount -t debugfs none /sys/kernel/debug
echo 1 > /sys/kernel/debug/tracing/events/msm_bus/
	bus_update_request/enable
cat /sys/kernel/debug/tracing/trace_pipe

CRs-Fixed: 622642
Change-Id: I9bd9eab8338c661ea170250a321eac7e40540fa6
Signed-off-by: Alok Chauhan <alokc@codeaurora.org>