clk: add versatile ICST307 driver

The ICST307 VCO clock has a shared driver in the ARM
architecture. This patch provides a wrapper into the common
clock framework so we can use the implementation in the
ARM architecture without duplicating the code until all
ARM platforms using this VCO are moved over. At that point
we can merge the driver from the ARM platform into the
generic file altogether.

Cc: Russell King <linux@arm.linux.org.uk>
Cc: Mike Turquette <mturquette@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[mturquette@linaro.org: removed versatile Kconfig]
Signed-off-by: Mike Turquette <mturquette@linaro.org>