[MIPS] Malta: include <linux/cpu.h> instead of <asm/cpu.h>

The checkpatch.pl script reported a few warnings about header files.
This patch fixes these warnings.

Compile-tested using the default Malta config.

No functional changes introduced.

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