sh: Fix OFFCHIP_IRQ_BASE compile error.

HP6xx uses OFFCHIP_IRQ_BASE to know the base irq number where non
cpu interrupts should start. This define was in irq.h before, but
since rework got lost. It really belongs inside hd64461.h since
the hp6xx wont work without it.

Signed-off-by: Kristoffer Ericson <Kristoffer.Ericson@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>