powerpc: Add POWER9 cputable entry

Add a cputable entry for POWER9.  More code is required to actually
boot and run on a POWER9 but this gets the base piece in which we can
start building on.

Copies over from POWER8 except for:
- Adds a new CPU_FTR_ARCH_300 bit to start hanging new architecture
   features from (in subsequent patches).
- Advertises new user features bits PPC_FEATURE2_ARCH_3_00 &
  HAS_IEEE128 when on POWER9.
- Drops CPU_FTR_SUBCORE.
- Drops PMU code and machine check.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>