Implement missing bus_space_subregion in kshim

Add missing function, used by exynos5_ehci driver.

Reviewed by: hselasky
Submitted by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Sponsored by: Juniper Networks Inc.
Differential Revision: https://reviews.freebsd.org/D4141