irqdomain: Eliminate dedicated radix lookup functions

In preparation to remove the slow revmap path, eliminate the public
radix revmap lookup functions.  This simplifies the code and makes the
slowpath removal patch a lot simpler.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Rob Herring <rob.herring@calxeda.com>