ppc4xx: Add lcd4_lwmon5 support

This patch adds the fast booting LWMON5 derivat "lcd4_lwmon5".
Its a stripped down version of the full blown lwmon5 support,
without ECC, USB, POST and some other stuff. It used the newly
introduced SPL infrastrucure for SPL from NOR flash booting
on the PPC4xx.

By setting the environment variable "boot_os" to "yes", Linux
will be started from the SPL version. If not, the "normal"
U-Boot will be started.

Signed-off-by: Stefan Roese <sr@denx.de>