arm64: Add ftrace support

This patch implements arm64 specific part to support function tracers,
such as function (CONFIG_FUNCTION_TRACER), function_graph
(CONFIG_FUNCTION_GRAPH_TRACER) and function profiler
(CONFIG_FUNCTION_PROFILER).

With 'function' tracer, all the functions in the kernel are traced with
timestamps in ${sysfs}/tracing/trace. If function_graph tracer is
specified, call graph is generated.

The kernel must be compiled with -pg option so that _mcount() is inserted
at the beginning of functions. This function is called on every function's
entry as long as tracing is enabled.
In addition, function_graph tracer also needs to be able to probe function's
exit. ftrace_graph_caller() & return_to_handler do this by faking link
register's value to intercept function's return path.

More details on architecture specific requirements are described in
Documentation/trace/ftrace-design.txt.

Reviewed-by: Ganapatrao Kulkarni <ganapatrao.kulkarni@cavium.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: 819e50e25d0ce8a75f5cba815416a6a8573655c4
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


[pratikp@codeaurora.org: resolve merge conflicts]

Conflicts:
	arch/arm64/kernel/arm64ksyms.c

Change-Id: I0795f84ef58b5741fc1a0103cb1b32e5621e5e3e
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>