ARM: at91/soc: Introduce register_devices callback

Some core devices should be registered by the SoC itself rather than by every
board using this SoC. Introduce a register_devices callback that should be
called during the init_machine in order to do that.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>