Provide Genode::cache_coherent() consistently

It got lost on base-foc and also was not put into Genode namespace in
the default implementation.

repos/base-foc/lib/mk/base-foc.inc

@@ -3,6 +3,7 @@ include $(BASE_DIR)/lib/mk/base.inc
 LIBS   += base-foc-common syscall-foc cxx
 
 SRC_CC += cap_map_remove.cc cap_alloc.cc
+SRC_CC += cache.cc
 SRC_CC += thread_start.cc
 SRC_CC += signal_transmitter.cc signal.cc
 SRC_CC += stack_area_addr.cc

repos/base/src/lib/base/cache.cc

@@ -17,5 +17,5 @@
  * This function needs to be implemented only for base platforms with ARM
  * support right now, so the default implementation does nothing.
  */
-void cache_coherent(Genode::addr_t, Genode::size_t) { }
+void Genode::cache_coherent(Genode::addr_t, Genode::size_t) { }