arch: Introduce S2RAM interface

Add a new API used by arch to implement suspend-to-RAM (S2RAM).

The API is composed by a single function to save the CPU context on
suspend.

A CPU context is the arch-specific set of registers that must be
preserved on power-off (in retained RAM) to be able to resume the
execution from the point it was suspended without going through the
whole kernel startup stage.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>