qfw: Switch to CONFIG_SYS_LOAD_ADDR from CONFIG_LOADADDR

qfw: Switch to CONFIG_SYS_LOAD_ADDR from CONFIG_LOADADDR

All platforms define CONFIG_SYS_LOAD_ADDR, but only some define
CONFIG_LOADADDR.  Very very rarely are these not the same address, and
qemu-ppce500 is one such case.  However, based on reading the history of
the code, this mismatched value was simply a copy-paste from other
PowerPC platforms where it is this unused currently.  Switch the code to
use CONFIG_SYS_LOAD_ADDR and update the documentation.

Cc: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>