MIPS: BCM63XX: setup the HSSPI clock rate

Properly set up the HSSPI clock rate depending on the SoC's PLL rate.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6177/