MIPS: Enable GENERIC_CPU_AUTOPROBE

MIPS: Enable GENERIC_CPU_AUTOPROBE

Add missing macros and methods that are required by
CONFIG_GENERIC_CPU_AUTOPROBE: MAX_CPU_FEATURES, cpu_have_feature(),
cpu_feature().

Also set a default elf platform as currently it is not set for most MIPS
platforms resulting in incorrectly specified modalias values in cpu
autoprobe ("cpu:type:(null):feature:...").

Export 'elf_hwcap' symbol so that it can be accessed from modules that
use module_cpu_feature_match()

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15395/

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