target/mips: add Octeon CvmCount RDHWR support

Octeon exposes CvmCount through RDHWR register 31. Add the Octeon-only
decode path, enable the corresponding HWREna bit for linux-user, and use
an unsigned mask when checking HWREna so bit 31 is handled safely.

For user-mode emulation, return host ticks as a monotonic counter source
suitable for existing Octeon userspace code. In system mode, fall back to
the existing CP0 Count value.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Message-ID: <20260608-mips-octeon-missing-insns-v2-v16-20-daef7a0d8b04@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>