MIPS: Make do_IRQ() available for assembly callers

MIPS: Make do_IRQ() available for assembly callers

As from commit 8f99a1626535 ("MIPS: Tracing: Add IRQENTRY_EXIT section
for MIPS") do_IRQ() is not a macro anymore and can be invoked directly
from assembly code again, however its `asmlinkage' annotation has never
been brought back from the previous removal of the function with commit
187933f23679 ("[MIPS] do_IRQ cleanup").

Since calling the function directly from assembly code has a performance
advantage, add the annotation back so that the DEC platform can make use
of this again.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>