PAPR will define a new ibm,pi-features bit which says that doorbells
should not be used even on architectures where they exist. This could be
because they are emulated and slower than using the interrupt controller
directly for IPIs.

Wire this bit into the pi-features parser to clear CPU_FTR_DBELL, and
ensure CPU_FTR_DBELL is not in CPU_FTRS_ALWAYS.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240207035220.339726-2-npiggin@gmail.com