dt-bindings: Add bitmain vendor prefix

Bitmain (https://www.bitmain.com) is a vendor of cryptocurrency
hardware.

Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -53,6 +53,7 @@ axentia	Axentia Technologies AB
 axis	Axis Communications AB
 bananapi BIPAI KEJI LIMITED
 bhf	Beckhoff Automation GmbH & Co. KG
+bitmain	Bitmain Technologies
 boe	BOE Technology Group Co., Ltd.
 bosch	Bosch Sensortec GmbH
 boundary	Boundary Devices Inc.