clk: provide prepare/unprepare functions

As discussed previously, there's the need on some platforms to run some
parts of clk_enable() in contexts which can schedule.  The solution
which was agreed upon was to provide clk_prepare() and clk_unprepare()
to contain this parts, while clk_enable() and clk_disable() perform
the atomic part.

This patch provides a common definition for clk_prepare() and
clk_unprepare() in linux/clk.h, and provides an upgrade path for
existing implementation and drivers: drivers can start using
clk_prepare() and clk_unprepare() once this patch is merged without
having to wait for platform support.  Platforms can then start to
provide these additional functions.

Eventually, HAVE_CLK_PREPARE will be removed from the kernel, and
everyone will have to provide these new APIs.

Acked-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>