Accton powerpc platforms: Adjust u-boot boot delay to 10secs

The platforms are including:
- AS5710_54X
- AS6700_32X
- AS6701_32X
- AS6710_32X
- AS7700_32X

u-boot boot delay in these platforms were not consistent in the
past.  We decided to follow ONIE's original design recently.  So
that these boot delay patches in machines are removed now.