ARM: integrator: move restart to the device tree

Using the augmented reset driver for the Versatile family,
we can move the reset handling for the Integrator out of the
machine. We add a "syscon" attribute to the core module, and
access the syscon registers using this handle. We need to
select SYSCON, POWER, POWER_RESET and POWER_RESET_VERSATILE
in order for the restart functionality to always be
available on all systems (it should not be optional).

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>