[POWERPC] clk.h interface for platforms

This provides an implementation of the <linux/clk.h> interface for
arch/powerpc using a set of function pointers in clk_functions.
Platforms that want to support this interface should fill
clk_functions and select CONFIG_PPC_CLOCK in Kconfig.

Signed-off-by: Domen Puncer <domen.puncer@telargo.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>