powerpc/qe: update risc allocation for QE

Change the RISC allocation to macros instead of enum, add function to read
the number of risc engines from the new property "fsl,qe-num-riscs" under
the qe node in dts. Add new property "fsl,qe-num-riscs" description in
qe.txt

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>