arm64: ftrace: (bugfix) synced with ftcace interface change

ftrace_init() failed since ftrace_dyn_arch_init() doesn't initialize
the argument to null.
This bug comes in only if arm64 ftrace is back-ported as
ftrace_dyn_arch_init() interface has been changed in the merge window of
3.15.

Change-Id: I43f3c9969ef3b07e134ad14342b0aae1378f3573
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>