Documentation: dt: Add bindings for Secure-only devices

The existing device tree bindings assume that we are only trying to
describe a single address space with a device tree (for ARM, either
the Normal or the Secure world). Some uses for device tree need to
describe both Normal and Secure worlds in a single device tree. Add
documentation of how to do this, by adding extra properties which
describe when a device appears differently in the two worlds or when
it only appears in one of them.

The binding describes the general principles for adding new
properties describing the secure world, but for now we only need a
single new property, "secure-status", which can be used to annotate
devices to indicate that they are only visible in one of the two
worlds.

The primary expected use of this binding is for a virtual machine
like QEMU to describe the VM layout to a TrustZone aware firmware
(which would then use the secure-only devices itself, and pass the DT
on to a kernel running in the non-secure world, which ignores the
secure-only devices and uses the rest).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>