powerpc/qe: update QE Serial Number

The latest QE chip may have more Serial Number(SNUM)s of thread to use. We
will get the number of SNUMs from device tree by reading the new property
"fsl,qe-num-snums", and set 28 as the default number of SNUMs so that it is
compatible with the old QE chips' device trees which don't have this new
property. The macro QE_NUM_OF_SNUM is defined as the maximum number in QE
snum table which is 256.

Also we update the snum_init[] array with 18 more new SNUMs which are
confirmed to be useful on new chip.

Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>
Acked-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>