dt-bindings: reset: Updated binding for Versal reset driver

dt-bindings: reset: Updated binding for Versal reset driver

Added documentation and Versal reset indices to describe
about Versal reset driver bindings.
In Versal all reset indices includes Class, SubClass, Type, Index
information whereas class refers to clock, reset, power etc.,
Underlying firmware in Versal have such classification and expects
the ID to be this way.
[13:0] - Index bits
[19:14] - Type bits
[25:20] - SubClass bits
[31:26] - Class bits.

Signed-off-by: Sai Krishna Potthuri <lakshmi.sai.krishna.potthuri@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>