MIPS: Add basic CPUFreq options.

This patch adds basic options for MIPS CPUFreq support.

Since the cp0 timer's frequency is based on the processor clockrate it can
not be used with CPUFReq; an additional external timer is required.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: cpufreq@vger.kernel.org,
Cc: Dave Jones <davej@redhat.com>,
Cc: Dominik Brodowski <linux@dominikbrodowski.net>,
Cc: yanh@lemote.com
Cc: huhb@lemote.com,
Patchwork: http://patchwork.linux-mips.org/patch/659/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>