mx6: fix build due to divide u64/u32

This could be fixed by using do_div, but I don't see any reason
for freq to be a u64.  decode_pll() returns a u32, and the
function mxc_get_pll_pdf() also returns a u32.  Therefore by
typing freq as a u32 we remove compilation errors like

libgcc.a(_udivdi3.o) uses VFP register arguments, u-boot does not