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>