powerpc/64s: Refine feature sets for little endian builds

This reduces vmlinux text size by 1kB and data by 1.5kB with a small
build!

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Add the recently added CPU_FTRS_POWER9_DD2_2 to the little
      endian possible mask as noticed by Nick.]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>