[platform][zynq] initial implementation of some clock routines

-Mostly introspection at this point
-Move target specific jam table to early platform init

target/zybo/rules.mk

@@ -11,7 +11,7 @@ GLOBAL_INCLUDES += \
 	$(LOCAL_DIR)/include
 
 GLOBAL_DEFINES += \
-	TIMER_CLOCK_FREQ=325000000 \
+	EXTERNAL_CLOCK_FREQ=50000000 \
 	WITH_STATIC_HEAP=1 \
 	HEAP_START=0x00100000 \
 	HEAP_LEN=0x1ff00000

target/zybo/target.c

@@ -20,15 +20,12 @@
  * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
  * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
-extern int ps7_init(void);
 
 void target_early_init(void)
 {
-    ps7_init();
 }
 
 void target_init(void)
 {
 }
 
-