rockchip: rk3288: Add reset reason detection

Sometimes it's helpful to know the reset reason caused in the SoC.
Add reset reason detection for the RK3288 SoC.
This will set an environment variable which represents the reset reason.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>