of: clean-up ifdefs in of_irq.h

Much of of_irq.h is needlessly ifdef'ed. Clean this up and minimize the
amount ifdef'ed code. This fixes some  build warnings when CONFIG_OF
is not enabled (seen on i386 and x86_64):

include/linux/of_irq.h:82:7: warning: 'struct device_node' declared inside parameter list [enabled by default]
include/linux/of_irq.h:82:7: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]
include/linux/of_irq.h:87:47: warning: 'struct device_node' declared inside parameter list [enabled by default]

Compile tested on i386, sparc and arm.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>