rockchip: rk3368: dts: add sgrf node

We will to drop device security temporarily (until the ATF initialises
it fully) from the TPL/SPL stage: this requires access to some
registers in the SGRF.

This adds the sgrf node to the rk3368.dtsi, so we can then bind a
syscon device onto it and access its memory ranges.

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