mips: ralink: get cpu rate from new driver code

At very early stage on boot, there is a need to set 'mips_hpt_frequency'.
This timer frequency is a half of the CPU frequency. To get clocks properly
set we need to call to 'of_clk_init()' and properly get cpu clock frequency
afterwards. Depending on the SoC, CPU clock index and compatible differs, so
use them to get the proper clock frm the clock provider. Hence, adapt code
to be aligned with new clock driver.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>