MIPS: Introduce set_elf_platform() helper function

MIPS: Introduce set_elf_platform() helper function

Replace these sequences:

if (cpu == 0)
	__elf_platform = "foo";

with a trivial inline function.

Signed-off-by: Robert Millan <rmh@gnu.org>
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Cc: wu zhangjin <wuzhangjin@gmail.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/2304/
Patchwork: https://patchwork.linux-mips.org/patch/2374/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>