[aon_timer] Initial commit of AON Timer docs

Just a basic translation of the proposal doc into markdown, plus an
initial draft of the HJSON spec.

Signed-off-by: Tom Roberts <tomroberts@lowrisc.org>
diff --git a/util/build_docs.py b/util/build_docs.py
index e9f065e..9f4244f 100755
--- a/util/build_docs.py
+++ b/util/build_docs.py
@@ -55,6 +55,7 @@
     # Pre-generate register and hwcfg fragments from these files.
     "hardware_definitions": [
         "hw/ip/aes/data/aes.hjson",
+        "hw/ip/aon_timer/data/aon_timer.hjson",
         "hw/top_earlgrey/ip/alert_handler/data/autogen/alert_handler.hjson",
         "hw/ip/entropy_src/data/entropy_src.hjson",
         "hw/ip/csrng/data/csrng.hjson",