scripts: dts: gen_defines: add DT_DEBRACKET_INTERNAL helper

Add a utility macro used to remove brackets from around a single
argument. While __DEBRACKET exists in util_internal.h, this change makes
DT independent (otherwise we should include util_internal.h, which is
another option).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>