of/microblaze: strip out of_irq_workarounds code

Microblaze doesn't have any legacy workaround in the device tree irq
mapping data.  All of the of_irq_workarounds stuff can be dropped

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>