powerpc/85xx: rename CONFIG_SYS_TEXT_BASE to CONFIG_SYS_MONITOR_BASE

Use CONFIG_SYS_MONITOR_BASE instead of CONFIG_SYS_TEXT_BASE in early
init code so we can share the same code with NAND or NOR boot and not
have additional ifdefs in here.

Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>