dtb symbols (__dtb_start, __dtb_end) are already defined using
RO_DATA_SECTION. This duplicated dtb section is a leftover used by
Kalray internal tools to patch the dtb when loading the elf on a
target. Remove it to keep only one symbol definition.

Signed-off-by: Clement Leger <cleger@kalray.eu>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>