siemens,am33x: adjust mtd partition

Use one mtd partition for rootfs and configuration by
means of ubi volumes and get rid of configuration partition.
We can use partition layout for both 256MB and 512MB flash.

Signed-off-by: Samuel Egli <samuel.egli@siemens.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Roger Meier <r.meier@siemens.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Heiko Schocher <hs@denx.de>