subsys/settings: add a function to save subtree

When running multiple subsystems or applications on a MCU, the usual
strategy is to use different settings subtrees.

The API provides a way to load or commit a subtree, to avoid adding
dependencies between subsystems or applications, but lacks the way to
save a subtree only. Fix that by adding a settings_save_subtree()
function.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>