clk: convert ARM RealView to common clk

This converts the ARM RealView machine over to using the common
clock. The approach is similar to the one used for the Integrator,
and we're reusing the ICST wrapper code.

We have to put the clock intialization in the timer init function
for the clocks to be available when initializing the timer,
keeping them in early_init() is too early for the common clk.

Since we now have to go down and compile drivers/clk/versatile
a CONFIG_COMMON_CLK_VERSATILE symbol has been added so the proper
code gets compiled into the kernel for either machine. A leftover
CLK_VERSATILE in the Integrator Kconfig was fixed up to use
the new symbol as well.

Tested on ARM RealView PB1176.

Cc: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>