[keymgr_dpe, testutils] Updates the testutils for earlgrey

To ensure backwards compatibility, the file has been split into two
sections due to the different boot processes. The first part covers
the boot process for darjeeling and the second part covers the boot process
for earlgrey. This `if define ...` should either be replaced with two
testutils files for the keymgr_dpe (linked by Bazel according to the top)
or with a finer granularity of defines (e.g. function-wise)

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

Author: rroth-lowrisc