iwlwifi: mvm: Add mem debugfs entry

In order to access cached/paged memory, there are a couple of firmware
commands (one for UMAC and one for LMAC) that let the host access memory
and registers indirectly. Since this is done by the firmware on behalf
of the host, even if memory is paged out or cached, the host will
retrieve the memory as the firmware sees it (paged out memory will get
paged in).

Export this mechanism via a debugfs entry for both read and write
access.

WARNING: This mechanism has no protections at all. Invalid addresses may
crash or hang the firmware. Writing to arbitrary memory also comes with
no guarantees.

Signed-off-by: Ido Yariv <idox.yariv@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>