powerpc: Update ISA versions to mention e5500/e6500

Add the NXP (nee Freescale) e5500 and e6500 to the ISA versions
documentation.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220803063228.1250030-1-mpe@ellerman.id.au