target-ppc: Copy and split gen_spr_7xx() for 970

This stops using 7xx common SPRs init function and adds separate set
of helpers for 970.

This does not copy ICTC SPR as neither 970 manual nor PowerISA mention it.

This defines 970/book3s PMU SPRs constants as they differs from the ones
used for 7XX.

This creates 2 helpers for PMU SPRs, one for supermode privileged SPRs and
one for user privileged SPRs as "sup" versions can be shared across
the family while "user" versions will behave different starting POWER8
(which will be addressed later).

This allows writing to Uxxxx SPRs from supermode. spr_write_ureg() is
implemented for this as a copy of already existing spr_read_ureg().

This allows writing to supervisor's SIAR - it used to be disabled
when gen_spr_7xx() was used.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
