Get CPU port dynamically in fabric interpreter

Different HW platforms have different CPU ports, here we allow using
the same interpreter with multiple platform.

Change-Id: I0895d4d3e11560c75aca3fa908ca38759b597d67
diff --git a/pipelines/fabric/src/main/resources/p4c-out/fabric-spgw/bmv2/default/cpu_port.txt b/pipelines/fabric/src/main/resources/p4c-out/fabric-spgw/bmv2/default/cpu_port.txt
new file mode 100644
index 0000000..ace9d03
--- /dev/null
+++ b/pipelines/fabric/src/main/resources/p4c-out/fabric-spgw/bmv2/default/cpu_port.txt
@@ -0,0 +1 @@
+255