dt-bindings: Amlogic: add the documentation for the SECBUS2 registers

The Meson8/Meson8b/Meson8m2 SoCs have a register bank called SECBUS2 which
contains registers for various IP blocks such as pin-controller bits for
the BSD_EN and TEST_N GPIOs as well as some AO ARC core control bits.
The registers can be accessed directly when not running in "secure mode".
When "secure mode" is enabled then these registers have to be accessed
through secure monitor calls.

So far these SoCs are always known to boot in "non-secure mode".
Add a binding documentation using syscon (as these registers are shared
across different IPs) for the SECBUS2 registers.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20210102205904.2691120-3-martin.blumenstingl@googlemail.com