pcu_l1_if_phy: add new PHY API function to disconnect PDCH

There is a function l1if_connect_pdch, but no complementary function
like we have it with l1if_open_pdch and l1if_close_pdch. The reason for
this is that the PHY implementations that rely on a femtocell DSP do not
need to disconnect the pdch explcitly. However, the planned support for
the E1 based Ercisson RBS CCU will require an explicit disconnect. So
lets add a function call for this.

Change-Id: Ied88f3289bda87c48f5f9255c4591470633cc805
Related: OS#5198
diff --git a/tests/ms/MsTest.cpp b/tests/ms/MsTest.cpp
index cbafee9..6a770c9 100644
--- a/tests/ms/MsTest.cpp
+++ b/tests/ms/MsTest.cpp
@@ -661,6 +661,7 @@
 extern "C" {
 void l1if_pdch_req() { abort(); }
 void l1if_connect_pdch() { abort(); }
+void l1if_disconnect_pdch() { abort(); }
 void l1if_close_pdch() { abort(); }
 void l1if_open_pdch() { abort(); }
 }