dt-bindings: Amlogic: add documentation for the SoC info register areas

There are three register areas which contain information about the SoC
version and revision:
- the assist registers contain the SoC's "major version" which encodes
  the SoC generation and part number. this is available on Meson6,
  Meson8 and Meson8b SoCs.
- the bootrom register contains at least the SoCs "misc version". this
  is avilable on Meson6, Meson8 and Meson8b
- the analog top registers contain information about the SoC revision.
  this is only available on Meson8 and Meson8b

Not much else is currently known about these registers.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>