hw/nvram/Kconfig: Add an entry for the NMC93xx EEPROM

The NMC93xx EEPROM is only used by few NIC cards and the
Am53C974 SCSI controller.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191231183216.6781-13-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>