[MIPS] Alchemy: fix IRQ bases

Do what the commits commits f3e8d1da and
9d360ab4 failed to achieve -- actually
convert the Alchemy code to irq_cpu.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>