arm64: Remove cpu_soft_reset() prototype

Commit 8e890713("Revert "arm64: kill flush_cache_all()"") added back
flush_cache_all but it also had the side effect of bringing back
cpu_soft_reset() prototype that had been subsequently moved elsewhere.

Change-Id: Ic0dedbad1aeba1b4fbde3f5dc8dd28f5ce830265
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>