docs: andes-ae350.md: Update ae350 documentation for fdt driver support

We update ae350 documentation to add details about platform device tree.
The nodes and their properties must be provided to properly initialize
data of underlying hardware and access their mmio registers.

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