[sw/device] Consider adding base/math.h

This is a follow of PR [lowrisc/opentitan#5973]. The sw/device codebase has many instances of ad-hoc rounding and alignment operations. This issue is to track the introduction of a math module providing a canonical implementation of such functions.

Start by reading the follow-up context in PR lowrisc/opentitan#5973 and surveying the ad-hoc rounding and alignment operations in sw/device. Define the scope for base/math.h and the canonical functions it should provide; the work is done when the project has an agreed, implemented math module covering that scope.

Written by the indexing model from the issue text.