platform: generic/andes: Enable generic platform support for AE350

We move andes directory to platform/generic as the necessary fdt
drivers are available, the users can enable the console, timer, ipi,
irqchip and reset devices by adding device tree nodes stated in the
docs/platform/andes-ae350.md.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Anup Patel <anup@brainfault.org>