sh: intc - remove redundant irq code for shmin

This patch removes redundant interrupt code for the shmin board which
is using a sh770x processor and 4 IRQ lines as individual interrupts
(IRQ-mode).

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>