config/arm: isolate generic build

config/arm: isolate generic build

Use generic configuration for the only build where it makes sense - the
generic build. For other builds, if we don't know either of implementer
ID or part number, the build is not supported.
Add part numbers to cross files where fallback to generic configuration
is assumed.

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Tested-by: Jerin Jacob <jerinj@marvell.com>
Tested-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Tested-by: Vimal Chungath <vcchunga@amazon.com>
Tested-by: Dharmik Thakkar <dharmik.thakkar@arm.com>