dm: x86: Create a driver for x86 interrupts

It seems likely that at some point we will want a generic interrupt uclass.
But this is a big undertaking as it involves unifying code across multiple
architectures.

As a first step, create a simple IRQ uclass and a driver for x86. This can
be generalised later as required.

Adjust pirq_init() to probe this driver, which has the effect of creating
routing tables and setting up the interrupt routing. This is a start
towards making interrupts fit better with driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

include/dm/uclass-id.h

@@ -37,6 +37,7 @@ enum uclass_id {
 	UCLASS_I2C_EEPROM,	/* I2C EEPROM device */
 	UCLASS_I2C_GENERIC,	/* Generic I2C device */
 	UCLASS_I2C_MUX,		/* I2C multiplexer */
+	UCLASS_IRQ,		/* Interrupt controller */
 	UCLASS_KEYBOARD,	/* Keyboard input device */
 	UCLASS_LED,		/* Light-emitting diode (LED) */
 	UCLASS_LPC,		/* x86 'low pin count' interface */