dm/VBS-U: implement read/write callbacks of modern common cfg

This patch implements the read/write callbacks for the registers in the
common configuration region. This region is implemented in the modern
MMIO bar.

Signed-off-by: Jian Jun Chen <jian.jun.chen@intel.com>
Reviewed-by: Hao Li <hao.l.li@intel.com>
Reviewed-by: Zhao Yakui <yakui.zhao@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

Author: chejianj