added a simple register example

examples/register/p4src/register.p4

@@ -0,0 +1,87 @@
+/*
+Copyright 2013-present Barefoot Networks, Inc. 
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+header_type ethernet_t {
+    fields {
+        dstAddr : 48;
+        srcAddr : 48;
+        etherType : 16;
+    }
+}
+
+header_type intrinsic_metadata_t {
+    fields {
+        mcast_grp : 4;
+        egress_rid : 4;
+        mcast_hash : 16;
+        lf_field_list: 32;
+    }
+}
+
+header_type meta_t {
+    fields {
+        register_tmp : 32;
+    }
+}
+
+metadata meta_t meta;
+
+parser start {
+    return parse_ethernet;
+}
+
+header ethernet_t ethernet;
+metadata intrinsic_metadata_t intrinsic_metadata;
+
+parser parse_ethernet {
+    extract(ethernet);
+    return ingress;
+}
+
+action _drop() {
+    drop();
+}
+
+action _nop() {
+}
+
+register my_register {
+    width: 32;
+    static: m_table;
+    instance_count: 16384;
+}
+
+action m_action(register_idx) {
+    register_read(meta.register_tmp, my_register, register_idx);
+    // TODO
+}
+
+table m_table {
+    reads {
+        ethernet.srcAddr : exact;
+    }
+    actions {
+        m_action; _nop;
+    }
+    size : 16384;
+}
+
+control ingress {
+    apply(m_table);
+}
+
+control egress {
+}