Add SPDX tags for chvgpio driver sources

Also move $FreeBSD$ keyword in header to BSD license

MFC after:	2 weeks

sys/dev/gpio/chvgpio.c

@@ -1,4 +1,6 @@
 /*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) 2017 Tom Jones <tj@enoti.me>
  * All rights reserved.
  *

sys/dev/gpio/chvgpio_reg.h

@@ -1,4 +1,6 @@
 /*-
+ * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+ *
  * Copyright (c) 2017 Tom Jones <tj@enoti.me>
  * All rights reserved.
  *
@@ -23,6 +25,7 @@
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
+ * $FreeBSD$
  */
 
 /*
@@ -40,7 +43,6 @@
  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  *
- * $FreeBSD$
  */