MIPS: Set ELF AT_PLATFORM string for BMIPS processors

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: Robert Millan <rmh@gnu.org>
Cc: 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/2300/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>