dt: bindings: Document ZynqMP DDRC in Synopsys documentation

Add information for ZynqMP DDRC which reports the single bit errors that
are corrected and the double bit errors that are detected.

Signed-off-by: Manish Narani <manish.narani@xilinx.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Rob Herring <robh@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
CC: amit.kucheria@linaro.org
CC: devicetree@vger.kernel.org
CC: leoyang.li@nxp.com
CC: linux-arm-kernel@lists.infradead.org
CC: linux-edac <linux-edac@vger.kernel.org>
CC: mchehab@kernel.org
CC: michal.simek@xilinx.com
CC: sudeep.holla@arm.com
Link: http://lkml.kernel.org/r/1540447621-22870-3-git-send-email-manish.narani@xilinx.com