target/ppc: removed VSCR from SPR registration

Since vscr is not an spr, its initialization was removed from the
spr registration functions, and moved to the relevant init_procs.

We may look into adding vscr to the reset path instead of the init
path (as suggested by David Gibson), but this looked like a good
enough solution for now.

Signed-off-by: Bruno Larsen (billionai) <bruno.larsen@eldorado.org.br>
Message-Id: <20210430193533.82136-6-bruno.larsen@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>