powerpc/85xx/qemu: Enable CONFIG_E500 and CONFIG_PPC_E500MC

The QEMU e500 board needs to enable CONFIG_E500 to correctly boot. QEMU
for ppc64 uses e5500/e6500 emulation, thus CONFIG_PPC_E500MC is required
as well.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Signed-off-by: Scott Wood <oss@buserror.net>