[POWERPC] Update default irq servers when boot cpu is removed

The xics code does update the default server information when the boot
cpu is removed.  This patch recognizes when the boot cpu is being
removed and updates the appropriate information based on the new 'boot
cpu'.

Failure to update this information can causes us to leave irqs pinned
to cpus that are being removed, especially when removing the boot cpu.
The cpu is removed from the kernel, but cpu dlpar remove operations
fail since we cannot return the cpu to the hypervisor.

Signed-off-by: Nathan Fonteno <nfont@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>