mvebu: mark all mtd partitions on GL.iNet GL-MV1000 read-only

mvebu: mark all mtd partitions on GL.iNet GL-MV1000 read-only

On this device, two of the three defined MTD partitions are
automatically set to read-only, since they do not end at an
erase/write block boundary.

In particular, the only partition remaining writable is the
one holding the u-boot bootloader.

Mark all of the partitions read-only, at least until a better
understanding of why the layout has been laid out this way is
gained.

Signed-off-by: Enrico Mioso <mrkiko.rs@gmail.com>