x86, of: Define irq functions to allow drivers/of/* to build on x86

- Define a stub irq_create_of_mapping for x86 as a stop-gap solution until
   drivers/of/irq is further along.
 - Define irq_dispose_mapping for x86 to appease of_i2c.c

These are needed to allow stuff in drivers/of/ to build on x86.  This stuff
will eventually get replaced; quoting Grant,

"The long term plan is to have the drivers/of/ code handling the mapping
intelligently like powerpc currently does."  But for now, just provide
these functions.

Signed-off-by: Andres Salomon <dilinger@queued.net>
LKML-Reference: <20101111214526.5de7121b@queued.net>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>