powerpc/powernv/idle: add a basic stop 0-3 driver for POWER10

This driver does not restore stop > 3 state, so it limits itself
to states which do not lose full state or TB.

The POWER10 SPRs are sufficiently different from P9 that it seems
easier to split out the P10 code. The POWER10 deep sleep code
(e.g., the BHRB restore) has been taken out, but it can be re-added
when stop > 3 support is added.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Pratik Rajesh <Sampat&lt;psampat@linux.ibm.com>
Tested-by: Vaidyanathan Srinivasan <svaidy@linux.ibm.com>
Reviewed-by: Pratik Rajesh <Sampat&lt;psampat@linux.ibm.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200819094700.493399-1-npiggin@gmail.com