target/ppc: renamed SPR registration functions

Renamed all gen_spr_* and gen_* functions specifically related to
registering SPRs to register_*_sprs and register_*, to avoid future
confusion with other TCG related code.

Signed-off-by: Bruno Larsen (billionai) <bruno.larsen@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210505155310.62710-2-bruno.larsen@eldorado.org.br>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>