docs: Add missing links in platform.md

The links to some of the platforms were missing in platform.md
file hence this patch.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>