Revert "mx6: fix build due to divide u64/u32"

The calculation in the mxc_get_pll_pfd() uses freq as u64, which is essential
since in the bottom it's multiplied by 18 and given that the PLL_BUS can be
528000000 then it might overflow

This reverts commit b7495f0347541eb068d11ff592d2d1f83d174ad9.