MPC8260: fixup device tree by property instead of path

cpu/mpc8260/cpu.c used to use do_fixup_by_path_u32() to update the
clock frequencies in the device tree, using a CPU path
"/cpus/OF_CPU", with OF_CPU beind defined in the board config file.

However, this does not work when one board config file (here:
MPC8260ADS.h) is intended to be used for several diffrent CPUs and
therefor contains a generic definition like "cpu@0", as the device
trees that will then be loaded will contain specific names like
"PowerPC,8272@0".

We switch to using do_fixup_by_prop_u32() instead, so we can search
for device_type="cpu", as it is done in other architectures, too.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>
Tested-by: Heiko Schocher <hs@denx.de>