The type 4 table generation code is very x86 centric today. Refactor things
out into the device model cpu class to allow the tables to get generated for
other architectures as well.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>