arm64: ftrace: Update stale comment

arm64: ftrace: Update stale comment

In commit:

  26299b3f6ba26bfc ("ftrace: arm64: move from REGS to ARGS")

... we folded ftrace_regs_entry into ftrace_caller, and
ftrace_regs_entry no longer exists.

Update the comment accordingly.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Florent Revest <revest@chromium.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230123134603.1064407-8-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>