MIPS: Alchemy: devboards: remove unneeded BCSR IRQ reg acc

Initially I had to write to both the MASK and ENABLE registers, otherwise
the CPLD would generate tons of spurious interrupts.  With the change to
the demux handler to disable the muxed line, it is now sufficient to
disable the interrupt by writing either the enable or mask register.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2865/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>