ppc4xx: Fix "chip_config" command for AMCC Arches

This patch fixes the "chip_config" command for I2C bootstrap EEPROM
configuration. First it changes the I2C bootstrap EEPROM address to
0x54 as this is used on Arches (instead of 0x52 on Canyonlands/
Glacier). Additionally, the NAND bootstrap settings are removed
for Arches since Arches doesn't support NAND-booting.

Signed-off-by: Stefan Roese <sr@denx.de>