[POWERPC] Fix performance regression in IRQ radix tree locking

When reworking the powerpc irq code, I figured out that we were using
the radix tree in a racy way. As a temporary fix, I put a spinlock in
there. However, this can have a significant impact on performances. This
patch reworks that to use a smarter technique based on the fact that
what we need is in fact a rwlock with extremely rare writers (thus
optimized for the read path).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>