Devkit8000: Fix build after introduction of GENERATED_GBL_DATA_SIZE

This patch fixes the issue by defining and using
CONFIG_SYS_INIT_RAM_SIZE and CONFIG_SYS_INIT_RAM_ADDR.

This patch adopts the
commit 31bfcf1c
from Steve Sakoman and Sandeep Paulraj on Devkit8000.

Signed-off-by: Thomas Weber <weber@corscience.de>