target/ppc: fix ISI fault cause for Radix MMU

target/ppc: fix ISI fault cause for Radix MMU

Fix Instruction Storage Interrupt (ISI) fault cause for Radix MMU,
when caused by missing PAGE_EXEC permission, to be
SRR1_NOEXEC_GUARD instead of DSISR_PROTFAULT.
This matches POWER9 hardware behavior.

Fixes: d5fee0bbe68 ("target/ppc: Implement ISA V3.00 radix page fault handler")
Signed-off-by: Leandro Lupori <leandro.lupori@eldorado.org.br>
Message-Id: <20220309192756.145283-1-leandro.lupori@eldorado.org.br>
Signed-off-by: Cédric Le Goater <clg@kaod.org>