NFC: trf7970a: Document the 'en2-rf-quirk' DT property

Current versions of the trf7970a have an erratum where
the EN2 pin must be kept low.  If it isn't kept low,
it will generate an RF field even when in passive
target mode.  To work around this issue, create the
'en2-rf-quirk' device tree property to indicate that
the trf7970a that the driver is using has this erratum.
Signed-off-by: NMark A. Greer <mgreer@animalcreek.com>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>

Documentation/devicetree/bindings/net/nfc/trf7970a.txt

@@ -16,6 +16,8 @@ Optional SoC Specific Properties:
 - vin-voltage-override: Specify voltage of VIN pin in microvolts.
 - irq-status-read-quirk: Specify that the trf7970a being used has the
   "IRQ Status Read" erratum.
+- en2-rf-quirk: Specify that the trf7970a being used has the "EN2 RF"
+  erratum.
 
 Example (for ARM-based BeagleBone with TRF7970A on SPI1):
 
@@ -36,6 +38,7 @@ Example (for ARM-based BeagleBone with TRF7970A on SPI1):
 		vin-voltage-override = <5000000>;
 		autosuspend-delay = <30000>;
 		irq-status-read-quirk;
+		en2-rf-quirk;
 		status = "okay";
 	};
 };