This implements support for ioremapping of register windows that
encapsulate clock control registers used by a struct clk, with
transparent sibling inheritance.

Root clocks at the top of a given topology often encapsulate the entire
register space of all of their sibling clocks, so this mapping can be
done once and handed down. A given clock enable/disable case maps out to
a single bit in a shared register, so this prevents creating multiple
overlapping mappings.

The mapping case breaks down in to a couple of different situations:

	- Sibling clocks without a specific mapping.
	- Root clocks without a specific mapping.
	- Any of sibling/root clocks with a specific mapping.

Sibling clocks with no specified mapping will grovel up the clock chain
and install the root clock mapping unconditionally at registration time.

Root clocks without their own mappings have a dummy BSS-initialized
mapping inserted that is handed down the chain just like any other
mapping. This permits all of the sibling clock ops to read/write using
the mapping offsets without any special configuration, enabling them to
not care whether access ultimately goes through translatable or
untranslatable memory.

Any clock with its own mapping will have the window initialized at
registration time and be ready for use by its clock ops. Failure to
establish the mapping will prevent registration, so no additional sanity
checks are needed. Sibling clocks that double as parents for the moment
will not propagate their mapping down, but this is easily tunable if the
need arises.

All clock mappings are kref refcounted, with each instance of mapping
inheritance incrementing the refcount.
Tested-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>