powerpc: Allow CPU selection of e300core variants

GCC supports -mcpu=e300c2 and -mcpu=e300c3

This patch gives the opportunity to tune kernel to one of
those two types.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>