powerpc/64s: Rename EXCEPTION_PROLOG_PSERIES_1 to EXCEPTION_PROLOG_2

As with the other patches in this series, we are removing the
"PSERIES" from the name as it's no longer meaningful.

In this case it's not simply a case of removing the "PSERIES" as that
would result in a clash with the existing EXCEPTION_PROLOG_1.

Instead we name this one EXCEPTION_PROLOG_2, as it's usually used in
sequence after 0 and 1.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>