x86/platform/uv: Constify static attribute_group struct

The only use of base_attr_group and hubless_base_attr_group is to pass
their addresses to sysfs_create_group() and sysfs_remove_group(), both
which takes pointers to const attribute_group structs. Make them const
to allow the compiler to put them in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Reviewed-by: Justin Ernst <justin.ernst@hpe.com>
Link: https://lore.kernel.org/r/20210605203807.60547-5-rikard.falkeborn@gmail.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>