[MIPS] Malta, Atlas, Sead: remove an extern from .c files

This patch moves the "extern" declaration for the function
mips_reboot_setup() from the board setup .c files to the
header file include/asm-mips/mips-boards/generic.h.

This fixes a warning produced by the checkpatch.pl script.

No functional changes introduced.

This was compile-tested by building the kernel for all
three boards affected by this change. All builds finished
successfully.

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