rockchip: rk3399-puma: add code to allow forcing a power-on reset

rockchip: rk3399-puma: add code to allow forcing a power-on reset

The reset circuitry in the RK3399 only resets 'almost all logic' when
a software reset is performed.  To make our software maintenance
easier in the future, we want to have the option (controlled by a DTS
property) to force all reset causes other than a power-on reset to
trigger a power-on reset via a GPIO trigger.

This adds the necessary support to the rk3399-puma (i.e. RK3399-Q7)
board-support and the documentation for the new property
(sysreset-gpio) within the /config-node.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>