powerpc/mpic: Fix irq distribution problem when MPIC_SINGLE_DEST_CPU

For the mpic with a flag MPIC_SINGLE_DEST_CPU, only one bit should be
set in interrupt destination registers.

The code is applicable to 64-bit platforms as well as 32-bit.

Signed-off-by: Zhao Chenhui <chenhui.zhao@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>