config/arm: add platform config option

config/arm: add platform config option

Add Arm SoC configuration sets to Arm meson.build and add an arch
agnostic meson option, 'platform', to select from these SoC
configurations for meson native builds. This is preferable to
specifying a cross file when doing aarch64 -> aarch64 builds, since the
cross file specifies the toolchain as well.

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Tested-by: Pavan Nikhilesh <pbhagavatula@marvell.com>