scripts/dts: Fix bug in dead /memreserve/ code

The /memreserve/ code would crash if it ever ran, because 'name' isn't
defined (seems to be some copy-paste here). There are no /memreserve/s
in Zephyr though, so it works out.

'name' seems to be the node name. Not sure what to put for a
/memreserve/, but make it '<memreserve>' to make it stand out.

Fixes a pylint warning.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>