platform: andes: Factor out is_andes() helper

We will need is_andes(45) in the following patch,
so factor out the code that parses marchid to make
it reusable for checking any Andes CPU variants.

Also improves the comment in ae350_hart_start().

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