powerpc: Make cpu_spec __ro_after_init

The cpu_spec is a struct holding various information about the CPU the
kernel is executing on. It's populated early in boot and must not change
after that.

In particular the cpu_features and mmu_features hold the set of
discovered CPU/MMU features and are used to set static keys for each
feature, and do binary patching of assembly. So any change to the
cpu_features/mmu_features later in boot will not be reflected in
the state of the static keys or patched code.

There is already logic to check that cpu_features/mmu_features don't
change, see check_features() in feature-fixups.c.

But as another layer of protection the entire cpu_spec should be read
only after init, annotate it as such.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231025012452.1985680-1-mpe@ellerman.id.au