powerpc: Add const to bin_attribute structures

Declare bin_attribute structures as const as they are only passed as an
argument to the function sysfs_create_bin_file. This argument is of
type const, so declare the structure as const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>