ARM: realview: select SP810 and ICST for the DT variant

The device tree boot for RealView need the SP810 system controller
(same as found on the Versatile Express) to set up the timers on the
board so the machine can tick. It further utilize the ICST307 through
its system controller for 6 other oscillators. We have to select these
from Kconfig or the machine does not boot.

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