Accton powerpc platforms: Revise the u-boot shell prompt

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

Revise the shell prompt to "LOADER=> " that will be consistent with
the one in other machines.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>