arm64: spe: Add introspection test

Probe the DTB and the ID registers to get information about SPE, then
compare the register fields with the valid values as defined by ARM DDI
0487F.b.

SPE is supported only on AArch64, so make the test exclusive to the
arm64 architecture.

[ Alexandru E: Removed aarch32 compilation support, added DTB probing,
	reworded commit, mostly cosmetic changes to the code ]

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>