vfio/platform: Cleanup Kconfig

Like vfio-pci, there's also a base module here where vfio-amba depends on
vfio-platform, when really it only needs vfio-platform-base.  Create a
sub-menu for platform drivers and a nested menu for reset drivers.  Cleanup
Makefile to make use of new CONFIG_VFIO_PLATFORM_BASE for building the
shared modules and traversing reset modules.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Link: https://lore.kernel.org/r/20230614193948.477036-3-alex.williamson@redhat.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>