platform/x86: intel_pmt_crashlog: Constify static attribute_group struct

The only use of pmt_crashlog_group is to assign its address to the
attr_grp field in the intel_pmt_namespace struct, which is a pointer to
const attribute_group. Make it const to allow the compiler to put it in
read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20210605203807.60547-3-rikard.falkeborn@gmail.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>