Replace register defines with the REG32 macro from registerfields.h in
the Cadence GEM device.

Signed-off-by: Luc Michel <luc.michel@amd.com>
Reviewed-by:  <sai.pavan.boddu@amd.com>
Message-id: 20231017194422.4124691-2-luc.michel@amd.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>