dt-parser: stricter typing
Add more type annotations to dt-parser and rework some bits to make mypy
happy.
Signed-off-by: 
Vincent Stehlé <vincent.stehle@arm.com>
Add more type annotations to dt-parser and rework some bits to make mypy
happy.
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>