mvebu: cortexa53: Enable Armada 37xx rWTM driver

mvebu: cortexa53: Enable Armada 37xx rWTM driver

Enabling Armada 37xx rWTM driver. This gives the kernel access to random
number generator, if provided by the rWTM firmware.

Note that the driver is called turris-mox-rwtm because it was initially
written for Turris MOX, but will also work on other Armada 37xx
platforms, if they use rWTM firmware from CZ.NIC [1].

It makes sense to have this driver built-in, so that kernel has access
to random number generator without needing to load any drivers.

[1] https://gitlab.nic.cz/turris/mox-boot-builder

Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andre Heider <a.heider@gmail.com>