mips: ar7: convert to CONFIG_COMMON_CLK

Perform a minimal conversion of the ar7 clock implementation to the common
clock framework. While the hardware can control the rates, this is left
unchanged, and all clocks are registered as fixed-rate or fixed-divider
clocks. Similarly, the clkdev lookup information is left unchanged but
moved from the table format into individual allocations.

There is a small increase in code size:

   text	   data	    bss	    dec	    hex	filename
4757116	 596640	  91328	5445084	 5315dc	vmlinux-before
4806159	 602360	  91344	5499863	 53ebd7	vmlinux-after

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