[POWERPC64LE,POWEPCSPE] set default kernel config for powerpc64le and powerpcspe variants

Default KERNCONF for powerpc64le should be GENERIC64, and powerpcspe should
select MPC85XXSPE

Reviewed by: bdragon,emaste
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D27257