powerpc: Update documentation of ISA versions for Power10

Update the CPU to ISA Version Mapping document to include Power10 and
ISA v3.1.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
[mpe: Make sure ISA reference is unique]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200827040556.1783-1-jniethe5@gmail.com