ARM: bL_switcher/trace: Add kernel trace trigger interface

This patch exports a bL_switcher_trace_trigger() function to
provide a means for drivers using the trace events to get the
current status when starting a trace session.

Calling this function is equivalent to pinging the trace_trigger
file in sysfs.

Signed-off-by: Dave Martin <dave.martin@linaro.org>

Conflicts:
	arch/arm/common/bL_switcher.c