powerpc: Add support for POWER8 directed doorbell interrupts

[Ian Munsie]

This patch series adds support for using directed doorbell interrupts for IPIs
between threads within a core, which is supported by POWER8.

This should avoid the overhead of using XICS to deliver an IPI within a core,
though delivering IPIs between threads of different cores must still use XICS
as before.

Please note that this series depends on this series posted by Michael Neuling,
as it adds additional relocation on exception vectors:
https://lists.ozlabs.org/pipermail/linuxppc-dev/2012-November/101910.html