hw/mem/nvdimm: Use Kconfig 'imply' instead of 'depends on'

hw/mem/nvdimm: Use Kconfig 'imply' instead of 'depends on'

Per the kconfig.rst:

  A device should be listed [...] ``imply`` if (depending on
  the QEMU command line) the board may or  may not be started
  without it.

This is the case with the NVDIMM device, so use the 'imply'
weak reverse dependency to select the symbol.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210511155354.3069141-2-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>