use AutoDoc for timer documentation

Not sure why nobody else saw this, but sometime in the last month's
patches sphinx started throwing an error when building docs for
the timer block. The problem is that the body and title are 'None'
and the doc code tries to invoke methods on None.

Changing the doc methodology to AutoDoc and explicitly creating the intro
section fixes this.

litex/soc/cores/timer.py

@@ -11,13 +11,14 @@ from migen import *
 
 from litex.soc.interconnect.csr import *
 from litex.soc.interconnect.csr_eventmanager import *
-from litex.soc.integration.doc import ModuleDoc
+from litex.soc.integration.doc import AutoDoc, ModuleDoc
 
 # Timer --------------------------------------------------------------------------------------------
 
-class Timer(Module, AutoCSR, ModuleDoc):
+class Timer(Module, AutoCSR, AutoDoc):
     with_uptime = False
-    """Timer
+    def __init__(self, width=32):
+        self.intro = ModuleDoc("""Timer
 
     Provides a generic Timer core.
 
@@ -50,8 +51,7 @@ class Timer(Module, AutoCSR, ModuleDoc):
 
     For both modes, the CPU can be advertised by an IRQ that the duration/period has elapsed. (The
     CPU can also do software polling with ``update_value`` and ``value`` to know the elapsed duration)
-    """
+    """)
-    def __init__(self, width=32):
         self._load = CSRStorage(width, description="""Load value when Timer is (re-)enabled.
             In One-Shot mode, the value written to this register specifies the Timer's duration in
             clock cycles.""")