[DRIVERS] clk: Make ARM reference board clocking more complete

The VExpress OSC clock implementation was missing ARM reference
clock implemetation. This patch adds it.

In addition, this patch also adds separate ICST library under
drivers/clk/versatile so that we don't depend on board support
code to provide ISCT library.

Signed-off-by: Anup Patel <anup@brainfault.org>