powerpc/64s: POWER10 CPU Kconfig build option

This adds basic POWER10_CPU option, which builds with -mcpu=power10.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220923033004.536127-1-npiggin@gmail.com