powerpc/8xx: do not select CONFIG_PPC_LIB_RHEAP

Since commit 0e6e01ff

 ("CPM/QE: use genalloc to manage CPM/QE
muram"), rheap is not used anymore.

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