ARM: integrator: convert to common clock

This converts the Integrator platform to use common clock
and the ICST driver. Since from this point not all ARM
reference platforms use the clock, we define
CONFIG_PLAT_VERSATILE_CLOCK and select it for all platforms
except the Integrator.

Open issue: I could not use the .init_early() field of the
machine descriptor to initialize the clocks, but had to
move them to .init_irq(), so presumably .init_early() is
so early that common clock is not up, and .init_machine()
is too late since it's needed for the clockevent/clocksource
initialization. Any suggestions on how to solve this is
very welcome.

Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[mturquette@linaro.org: use 'select' instead of versatile Kconfig]
Signed-off-by: Mike Turquette <mturquette@linaro.org>