arm64: stacktrace: rename unwinder functions

Rename unwinder functions for consistency and better naming.

	- Rename start_backtrace() to unwind_init().
	- Rename unwind_frame() to unwind_next().
	- Rename walk_stackframe() to unwind().

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

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Kalesh Singh <kaleshsingh@google.com> for the series.
Link: https://lore.kernel.org/r/20220413145910.3060139-5-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>