sys: util: Add clamp macro

Adds CLAMP macro to complement the current min/max macros, as well as a
gcc specific Z_CLAMP macro for single-evaluation expansion.

CLAMP combines the functionality of MIN and MAX, eliminating the
bug-prone usage of MIN(MAX(value, FLOOR), CEIL) found throughout the
codebase in every possible combination.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>