powerpc: Add "force config cmd line" Kconfig option

powerpc uses early_init_dt_scan_chosen() from common fdt code. By
enabling this option, the common code can take the built in
command line over the one that is comming from bootloader / DT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>