microblaze: Fix IRQ entry/exit ftracing

Function traces on Microblaze don't include IRQ entry and exit arrows,
i.e.

 0)               |                            memcpy_toiovec() {
 0)   ==========> |
 0)               |                              do_IRQ() {
 ...
 0)   <========== |
 0) ! 5414.000 us |                            }

...because do_IRQ() doesn't have the proper attributes.

Signed-off-by: Steven J. Magnani <steve@digidescorp.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>