[docs] Revise TODO list and update links

Updated the TODO list format and provided a link to the current TODO document.

Author: travisg

docs/todo.md

@@ -1,12 +1,6 @@
-# Random TODO list
+# TODO
 
-[github issue list](https://github.com/littlekernel/lk/issues)
+Current TODO list is in a google doc at [TODO](https://docs.google.com/document/d/16GU0JWIqEB--hj0jWiq-wKP8tmG5zXb1W49qBwp5GjQ/edit?usp=sharing).
 
-To fix:
-+   arm64 arch_ops.h - arch_cycle_count is bogus
-+   x86 arch_ops.h - switch to builtin atomics
-+   riscv - exception decode can be a negation test (top bit set)
-+   arm arch_ops.h left over pre-armv6 stuff
-+   see about expanding cycle count to a ulong
-+   lib console make read only blocks
-	
+[github issue list](https://github.com/littlekernel/lk/issues)
+[github pull requests](https://github.com/littlekernel/lk/pulls)