arch/powperpc: Fix start_align due to use of __ffs64() instead ffs64()

Incorrect DDR law was created in case of B4860qds after commit
2d2f490d. Return value of ffs64() differs from __ffs64(), eg.
    ffs64(0x80000000) = 0x20
    __ffs64(0x80000000) = 0x1f
As a result of this change, callers need to adjust the expected
return value by removing -1.

Signed-off-by: Ashish Kumar <Ashish.Kumar@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>