powerpc, 8xx: Properly set CPM frequency in the device tree

For processors whose core runs at twice the bus frequency,
the fallback frequency calculation in Linux provides a wrong
result. Therefore, U-boot needs to pass the correct value.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Heiko Schocher <hs@denx.de>