Move PDCH-related functions into separate files

The PDCH class and corresponding functions are rather self-contained and
independent from BTS implementation. Let's move them into separate file
to make bts.cpp more manageable. As additional benefit it allow us to
somewhat untangle all the different cross-dependent includes.

Change-Id: Ie05e25361e6741a81b024679f9675c98d4923683
Related: OS#1539
diff --git a/src/osmo-bts-litecell15/lc15_l1_if.c b/src/osmo-bts-litecell15/lc15_l1_if.c
index 8290a93..1df7941 100644
--- a/src/osmo-bts-litecell15/lc15_l1_if.c
+++ b/src/osmo-bts-litecell15/lc15_l1_if.c
@@ -34,6 +34,7 @@
 #include <lc15_l1_if.h>
 #include <gprs_debug.h>
 #include <pcu_l1_if.h>
+#include <pdch.h>
 #include <bts.h>
 
 extern void *tall_pcu_ctx;