spi: cadence_qspi: use "cdns,qspi-nor" as compatible

Linux uses "cdns,qspi-nor" as compatible string for the cadence
qspi driver, so change driver, docs and all device trees.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
diff --git a/arch/arm/dts/socfpga.dtsi b/arch/arm/dts/socfpga.dtsi
index 3144494..9d8c4eb 100644
--- a/arch/arm/dts/socfpga.dtsi
+++ b/arch/arm/dts/socfpga.dtsi
@@ -646,7 +646,7 @@
 		};
 
 		qspi: spi@ff705000 {
-			compatible = "cadence,qspi";
+			compatible = "cdns,qspi-nor";
 			#address-cells = <1>;
 			#size-cells = <0>;
 			reg = <0xff705000 0x1000>,