arm-bsp/u-boot: corstone1000: purge U-Boot specific DT nodes before Linux

Remove U-Boot specific DT nodes before passing the DT to Linux

This is needed to pass SystemReady IR 2.0 dt-schema tests

Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>