ppc4xx: Fix sys_get_info() for 405GP(r)

This patch assigns the correct EBC clock for 405GP(r) CPUs
to PPC4xx_SYS_INFO structure. Without this patch U-Boot
uses an uninitialized EBC clock in its startup message.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>