net: mvpp2: move from cpu-centric naming to "software thread" naming

The PPv2.2 IP has a concept of "software thread", with all registers
of the PPv2.2 mapped 8 times, for concurrent accesses by 8 "software
threads". In addition, interrupts on RX queues are associated to such
"software thread".

For most cases, we map a "software thread" to the more conventional
concept of CPU, but we will soon have one exception: we will have a
model where we have one TX interrupt per CPU (each using one software
thread), and all RX events mapped to another software thread
(associated to another interrupt).

In preparation for this change, it makes sense to change the naming
from MVPP2_MAX_CPUS to MVPP2_MAX_THREADS, and plan for 8 software
threads instead of 4 currently.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>