mips: ralink: convert to CONFIG_COMMON_CLK

ralink only has a very trivial clock implementation, with everything
being fixed clocks.

Convert it to CONFIG_COMMON_CLK to reduce the number of platforms
that rely on legacy clocks. Of course, the clocks really should
be read from the device tree instead, but this is a step into that
direction.

This adds about 50KB to the kernel image size, which is an unfortunate
increase, but not as bad as I had feared:

   text	   data	    bss	    dec	    hex	filename
3778560	1582216	  92256	5453032	 5334e8	vmlinux-vocore-before
3822148	1601192	  92304	5515644	 54297c	vmlinux-vocore-after
3870226	1644468	 200192	5714886	 5733c6	vmlinux-rt305x-before
3916727	1668404	 200240	5785371	 58471b	vmlinux-rt305x-after

Signed-off-by: Arnd Bergmann <arnd@arndb.de>