Documentation: dt: keystone: ti-sci: Add optional host-id parameter

Texas Instrument's System Control Interface (TISCI) permits
the ability for OSs running in virtual machines to be able to
independently communicate with the firmware without the need going
through an hypervisor.

The "host-id" in effect is the hardware representation of the
host (example: VMs locked to a core) as identified to the System
Controller. Hypervisors can either fill in appropriate host-ids in dt
used for each VM instance OR may use prebuilt blobs where the host-ids
are pre-populated, as appropriate for the OS running in the VMs.

This is introduced as an optional parameter to maintain consistency
with legacy device tree blobs.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>