linker: allocate devices in ROM

Pull request https://github.com/zephyrproject-rtos/zephyr/pull/24873


marked all the struct devices const.

However, the linker scripts were never updated to actually allocate
the struct devices in ROM. They're still in RAM, so the main benefit
of this work has not been realized.

Fix it.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>