powerpc/64: Sanitise user registers on interrupt in pseries, POWERNV

Cause pseries and POWERNV platforms to default to zeroising all potentially
user-defined registers when entering the kernel by means of any interrupt
source, reducing user-influence of the kernel and the likelihood or
producing speculation gadgets.
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221201071019.1953023-7-rmclure@linux.ibm.com

arch/powerpc/Kconfig

@@ -537,7 +537,7 @@ config HOTPLUG_CPU
 config INTERRUPT_SANITIZE_REGISTERS
 	bool "Clear gprs on interrupt arrival"
 	depends on PPC64 && ARCH_HAS_SYSCALL_WRAPPER
-	default PPC_BOOK3E_64
+	default PPC_BOOK3E_64 || PPC_PSERIES || PPC_POWERNV
 	help
 	  Reduce the influence of user register state on interrupt handlers and
 	  syscalls through clearing user state from registers before handling