ramoops: Add "max-reason" optional field to ramoops DT node

Currently, it is only possible to get kmsg dumps for panic and oops,
or just panic, via "no-dump-oops". With "max-reason" it is possible to
dump messages for other kmsg_dump events, for example emerg and shutdown.

Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Link: https://lore.kernel.org/lkml/20200515184434.8470-7-keescook@chromium.org/


Signed-off-by: Kees Cook <keescook@chromium.org>