mtd: Disable slram and phram when securelevel is enabled

mtd: Disable slram and phram when securelevel is enabled

The slram and phram drivers both allow mapping regions of physical
address space such that they can then be read and written by userland
through the MTD interface.  This is probably usable to manipulate
hardware into overwriting kernel code on many systems.  Prevent that
if securelevel is set.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic features/all/securelevel
Gbp-Pq: Name mtd-disable-slram-and-phram-when-securelevel-is-enabled.patch