android: persistent_trace: ftrace into persistent_ram

persistent_trace uses the ftrace infrastructure, but traces
into a persistent_ram buffer instead of the regular ftrace
ringbuffer.  After a reset or panic, the trace can be
decoded with cat /sys/kernel/debug/persistent_trace.

Signed-off-by: Colin Cross <ccross@android.com>

Change-Id: Ia6025ccc323599c7844e0783af0386d32ed7419e