[MIPS] unexport {allocate,free}_irqno

The following routines

allocate_irqno()
free_irqno()

seem not to be used outside of the core kernel code, hence
exporting these functions is pointless. This patch removes
the export.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>