MIPS: ath79: Correctly name the defines for the PLL_FB register
This register is named PLL_FB and is not a divider but a multiplier.
To make things less confusing rename the ARxxxx_PLL_DIV_SHIFT and
ARxxxx_PLL_DIV_MASK macros to ARxxxx_PLL_FB_SHIFT and
ARxxxx_PLL_FB_MASK.