docs: platform: Update AE350 and generic platform documentation

Update compile option and platform compatible string for AE350 and
add it to the generic platform list.

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