powerpc/kernel: Check features don't change after patching

Early in boot we binary patch some sections of code based on the CPU and
MMU feature bits. But it is a one-time patching, there is no facility
for repatching the code later if the set of features change.

It is a major bug if the set of features changes after we've done the
code patching - so add a check for it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>