configs: powerpc: Don't set CONFIG_ENV_ADDR for QorIQ SPIFLASH

Setting CONFIG_ENV_ADDR to something other than 0 stops gd->env_addr
from being allocated dynamically. When the environment is in SPI we need
it to be allocated as we can't use a direct memory mapped address.

Signed-off-by: Chris Packham <judge.packham@gmail.com>