drivers: Add AXI uclass

Add a uclass for AXI (Advanced eXtensible Interface) busses, and a
driver for the gdsys IHS AXI bus on IHS FPGAs.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/dm/uclass-id.h

@@ -43,6 +43,7 @@ enum uclass_id {
 	UCLASS_I2C_GENERIC,	/* Generic I2C device */
 	UCLASS_I2C_MUX,		/* I2C multiplexer */
 	UCLASS_IDE,		/* IDE device */
+	UCLASS_AXI,		/* AXI bus */
 	UCLASS_IRQ,		/* Interrupt controller */
 	UCLASS_KEYBOARD,	/* Keyboard input device */
 	UCLASS_LED,		/* Light-emitting diode (LED) */