GridSeed docs are actually wrong, pll_BS is bit 31, and pll_BP is probably bit 30. If you look at the frequency tables in cgminer, you'll see that for frequencies >= 500 MHz, the highest bit is set.
Actually the gridseed version of cgminer sets bit30 low for 250/400/450mhz, and high for 500mhz and above just as they specified it in the docs.