powerpc/dexcr: Add initial Dynamic Execution Control Register (DEXCR) support

ISA 3.1B introduces the Dynamic Execution Control Register (DEXCR). It
is a per-cpu register that allows control over various CPU behaviours
including branch hint usage, indirect branch speculation, and
hashst/hashchk support.

Add some definitions and basic support for the DEXCR in the kernel.
Right now it just

  * Initialises the DEXCR and HASHKEYR to a fixed value when a CPU
    onlines.
  * Clears them in reset_sprs().
  * Detects when the NPHIE aspect is supported (the others don't get
    looked at in this series, so there's no need to waste a CPU_FTR
    on them).

We initialise the HASHKEYR to ensure that all cores have the same key,
so an HV enforced NPHIE + swapping cores doesn't randomly crash a
process using hash instructions. The stores to HASHKEYR are
unconditional because the ISA makes no mention of the SPR being missing
if support for doing the hashes isn't present. So all that would happen
is the HASHKEYR value gets ignored. This helps slightly if NPHIE
detection fails; e.g., we currently only detect it on pseries.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
[mpe: Use simple values for DEXCR constants]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230616034846.311705-4-bgray@linux.ibm.com