clocksource: Sched clock source for Versatile Express

This patch adds a trival sched clock source using free
running, 24MHz clocked counter present in the ARM Ltd.
reference platforms (Versatile, RealView, Versatile
Express) System Registers block.

This code replaces the call in the VE machine code.

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