tests/qtest: Fix GMAC test to run on a machine in upstream QEMU

tests/qtest: Fix GMAC test to run on a machine in upstream QEMU

Fix the nocm_gmac-test.c file to run on a nuvoton 7xx machine instead
of 8xx. Also fix comments referencing this and values expecting 8xx.

Change-Id: Iabd0fba14910c3f1e883c4a9521350f3db9ffab8
Signed-Off-By: Nabih Estefan <nabihestefan@google.com>
Reviewed-by: Tyrone Ting <kfting@nuvoton.com>
Message-id: 20240208194759.2858582-2-nabihestefan@google.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: commit message tweaks]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>