[MIPS] Alchemy: Fix Au1x SD controller IRQ

With the introduction of MIPS_CPU_IRQ_BASE, the hardcoded IRQ number of
the au1100/au1200 SD controller(s) is no longer valid.

Signed-off-by: Manuel Lauss <mano@roarinelk.homelinux.net>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>