[POWERPC] PS3: Kexec support

Fixup the core platform parts needed for kexec to work on the PS3.
 - Setup ps3_hpte_clear correctly.
 - Mask interrupts on irq removal.
 - Release all hypervisor resources.
 - Create new routine ps3_shutdown_IRQ()

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>