dt-bindings: clock: meson8b: describe the embedded reset controller

The Amlogic Meson8/Meson8b/Meson8m2 clock controller provides some reset
lines. These are used for example to boot the secondary CPU cores.

This patch describes the reset controller which is embedded into the
clock controller on these SoCs.
A header file is provided which provides preprocessor macros for each
reset line (to make the .dts files easier to read).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>