powerpc: Fix _ALIGN_* errors due to type difference.

This avoid errors like

        unsigned int usize = 1 << 30;
        int size = 1 << 30;
        unsigned long addr = 64UL << 30 ;

        value = _ALIGN_DOWN(addr, usize); -> 0
        value = _ALIGN_DOWN(addr, size);  -> 0x1000000000

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>