powerpc: fix spelling mistake in Kconfig "seleted" -> "selected"

There is a spelling mistake in the help text of the Kconfig. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201207155420.172370-1-colin.king@canonical.com