mips: loongsoon2ef: remove private clk api

As platforms are moving to COMMON_CLK in general, loongson2ef
stuck out as something that has a private implementation but
does not actually use it except for setting the frequency of
the CPU itself from the loongson2_cpufreq driver.

Change that driver to call the register setting function directly
and remove the rest of the stub implementation.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>