MIPS: Loongson1B: use common clock infrastructure instead of private APIs

Use common clock infrastructure instead of private APIs.
1. Enable COMMON_CLK in the Kconfig.
2. Remove private clock APIs, which are replaced by the code in
   drivers/clk/clk-ls1x.c.
3. Modify header file for drivers/clk/clk-ls1x.c.

Signed-off-by: Kelvin Cheung <keguang.zhang@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/4431
Signed-off-by: John Crispin <blogic@openwrt.org>