powerpc/85xx: Fix compile errors if CONFIG_SYS_{BR,OR}0_PRELIM aren't set

Add ifdef protection in LBC code to handle the case in which
CONFIG_SYS_BR0_PRELIM and CONFIG_SYS_OR0_PRELIM arent defined for a
build.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>