mpc85xx: add mpic timer clock frequency to avoid boot log warning

mpc85xx: add mpic timer clock frequency to avoid boot log warning

MPIC timers are enabled in the config, however a devicetree clock setting
is missing causing the following error:
/soc@ffe00000/timer@41100: cannot get timer frequency.
/soc@ffe00000/timer@42100: cannot get timer frequency.

This patch adds the missing clock and avoids the error.
It's a functional copy of this code
do_fixup_by_compat_u32(blob, "fsl,mpic",
        "clock-frequency", get_bus_freq(0), 1);
in arch/powerpc/cpu/mpc85xx/fdt.c in the u-boot code.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44501 3c298f89-4303-0410-b956-a3cf2f4a3e73