powerpc/64s: Merge HV and non-HV paths for doorbell IRQ replay

This results in smaller code, and fewer branches. This relies on the
fact that both the 0xe80 and 0xa00 handlers call the same upper level
code, namely doorbell_exception().

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Mention we rely on the implementation of the 0xe80/0xa00 handlers]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>