target/mips: add Octeon ZUC COP2 helpers

Add the Octeon ZUC START and MORE helper operations and model the shared
state window used by the hardware interface. This covers the keystream
and MAC engine state, including the save-and-restore view that overlaps
the HSH/SHA3 bank.

Keep the LFSR words in the architectural HSH DAT input registers and the
runtime MAC/FSM/result state in the documented HASHIV window. The third
MAC lookahead word is generated on demand instead of being kept in a
non-architectural shadow slot.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Message-ID: <20260608-mips-octeon-missing-insns-v2-v16-6-daef7a0d8b04@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>