arm64: dts: librem5: protect some partitions of the nor-flash

These sections should be read only as they contain important data.

Signed-off-by: Angus Ainslie <angus@akkea.ca>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>