[device,base] document policy regarding 64-bit division

Signed-off-by: Amaury Pouly <amaury.pouly@opentitan.org>

Author: pamaury

sw/device/lib/base/README.md

@@ -9,3 +9,9 @@ This subtree provides headers and libraries known collectively as `libbase`, whi
 
 In general, a library exposing general utilities for working close to the hardware should live in this subtree: for example, a library providing `memcpy` and related symbols.
 A library for talking to a particular peripheral, like a UART port, is a non-example.
+
+## Policy regarding 64-bit division
+
+`libbase` does not provide the `__divdi3` and `__udivdi3` (signed/unsigned 64-bit division) symbols.
+The rationale is that users of the library may have different needs when it comes to the trade-off between the size and speed of the implementation.
+Instead, `libbase` provides `udiv64_slow` which is a relatively slow but compact unsigned 64-bit division.