clk: introduce CLK_ENABLE_HAND_OFF flag

Some clocks are critical to system operation (e.g. cpu, memory, etc) and
should not be gated until a driver that knows best claims such a clock
and expressly gates that clock through the normal clk.h api.

The typical way to handle this is for the clk driver or some other early
code to call clk_prepare_enable on this important clock as soon as it is
registered and before the clk_disable_unused garbage collector kicks in.

This patch introduces a formal way to handle this scenario that is
provided by the clk framework. Clk driver authors can set the
CLK_ENABLE_HAND_OFF flag in their clk data, which will cause the clk to
be enabled in clk_register(). Then when the first clk consumer driver
comes along and calls clk_get() & clk_prepare_enable(), the reference
counts taken during clk registration are transferred (or handed off) to
the clk consumer.

At this point handling the clk is the same as any other clock which as
not set the new CLK_ENABLE_HAND_OFF flag. In fact no changes to any
clock consumer driver are needed for this to work.

Change-Id: Ib5247f6bceb1f555c03103f061af089755b2de62
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Patch-mainline: patchwork.kernel.org @ 02/11/16, 9:19
Signed-off-by: Taniya Das <tdas@codeaurora.org>
Signed-off-by: David Dai <daidavid1@codeaurora.org>