[keymgr_dpe, sw] Add first version of the silicon driver

This commit introduces a first version of the keymgr_dpe sw in
the silicon creator lib. It covers the basic keymgr_dpe features.

Add source / header file **without** linking them in the
appropriate *BUILD* file as some dependencies do not exist yet!

Add an empty keymgr_dpe_unittest.cc file to avoid triggering error
during the top integration, tracked in Issue #30609.

Signed-off-by: Raphael Roth <rroth@lowrisc.org>

Author: rroth-lowrisc