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Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

libsel4vmmplatsupport: Update documentation

This commit refreshes the interface documentation in
libsel4vmmplatsupport. This commit patches in missing documentation
and additionally ensures all interface documentation conforms to
Doxygen style comments.

trivial: Style changed files

libsel4vmmplatsupport: arch include -> arch subdir

Moved the architecture specific headers, being unique header files
that only exist for a given architecture, into an 'arch'
subdirectory. Since this library can be used across architectures
this avoids polluting the root libsel4vmmplatsupport namespace and
adds further structure to distinguish different interfaces.