Instead of referencing NO_IRQ in platform.c, define some helper functions
in irq.c to call instead from platform.c.  Keep NO_IRQ usage local to
irq.c, and define NO_IRQ if not defined in headers.
Signed-off-by: NAndres Salomon <dilinger@queued.net>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>