drm/ast: Create threshold values for AST2600

The threshold value is used for AST2600 only.

Signed-off-by: KuoHsiang Chou <kuohsiang_chou@aspeedtech.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220117083643.41493-1-kuohsiang_chou@aspeedtech.com