[MIPS] Malta, Atlas: move an extern function declaration to the header file

This was compile-tested using default configs for the boards
affected by this change.

This patch does not introduce any functional changes.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>