doc: dt: document altera-passive-serial binding

Describe an altera-passive-serial devicetree entry, required features

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/devicetree/bindings/fpga/altera-passive-serial.txt b/Documentation/devicetree/bindings/fpga/altera-passive-serial.txt
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+Altera Passive Serial SPI FPGA Manager
+
+Altera FPGAs support a method of loading the bitstream over what is
+referred to as "passive serial".
+The passive serial link is not technically SPI, and might require extra
+circuits in order to play nicely with other SPI slaves on the same bus.
+
+See https://www.altera.com/literature/hb/cyc/cyc_c51013.pdf
+
+Required properties:
+- compatible: Must be one of the following:
+       "altr,fpga-passive-serial",
+       "altr,fpga-arria10-passive-serial"
+- reg: SPI chip select of the FPGA
+- nconfig-gpios: config pin (referred to as nCONFIG in the manual)
+- nstat-gpios: status pin (referred to as nSTATUS in the manual)
+
+Optional properties:
+- confd-gpios: confd pin (referred to as CONF_DONE in the manual)
+
+Example:
+       fpga: fpga@0 {
+               compatible = "altr,fpga-passive-serial";
+               spi-max-frequency = <20000000>;
+               reg = <0>;
+               nconfig-gpios = <&gpio4 9 GPIO_ACTIVE_LOW>;
+               nstat-gpios = <&gpio4 11 GPIO_ACTIVE_LOW>;
+               confd-gpios = <&gpio4 12 GPIO_ACTIVE_LOW>;
+       };