powerpc: Kconfig: Replace tabs with whitespaces

Replace tabs after keywords with whitespaces to be consistent.
Signed-off-by: Juerg Haefliger <juerg.haefliger@canonical.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220526065737.86370-2-juerg.haefliger@canonical.com