firmware: Add Turris Mox rWTM firmware driver

firmware: Add Turris Mox rWTM firmware driver

This adds a driver to communicate with the firmware running on the
secure processor of the Turris Mox router, enabling the kernel to
retrieve true random numbers from the Entropy Bit Generator and to read
some information burned into eFuses when device was manufactured:

and to
sign messages with the ECDSA private key burned into each Turris Mox
device when manufacturing.

This also adds support to read other information burned into eFuses:
 - serial number
 - board version
 - MAC addresses
 - RAM size
 - ECDSA public key (this is not read directly from eFuses, rather it
   is computed by the firmware as pair to the burned private key)

The source code of the firmware is open source and can be found at
https://gitlab.labs.nic.cz/turris/mox-boot-builder/tree/master/wtmi

The firmware is also able to, on demand, sign messages with the burned
ECDSA private key, but since Linux's akcipher API is not yet stable
(and therefore not exposed to userspace via netlink), this functionality
is not supported yet.

Link: https://lore.kernel.org/r/20190822014318.19478-3-marek.behun@nic.cz
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>