cli: Update database schema to support any arch

Add an "architecture" field to the SUT table, which contains whatever
`uname -m` returns. And make the CPU table much more generic. It now
contains a "desc" field which holds arch-specific JSON-encoded data.

The code can explicitly encode/decode aach64 and x86_64 arches, but it
would be trivial to add more and this can be done without changing the
schema.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>