src/drivers/uart: Add definition of FIFO enabled in IIR

Interrupt Identification Register (IIR) is a I/O read-access register.
Add definition of FIFO enabled for this register so that we can check
whether FIFO is enabled or not.

Signed-off-by: Tim Chu <Tim.Chu@quantatw.com>
Change-Id: I12e8566822693004418cf83cae466dc3e2d612c4
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68566
Reviewed-by: Jonathan Zhang <jonzhang@fb.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/drivers/uart/uart8250reg.h

@@ -24,6 +24,7 @@
 #define   UART8250_IIR_THRI	0x02 /* Transmitter holding register empty */
 #define   UART8250_IIR_RDI	0x04 /* Receiver data interrupt */
 #define   UART8250_IIR_RLSI	0x06 /* Receiver line status interrupt */
+#define   UART8250_IIR_FIFO_EN	0xC0 /* FIFO enabled */
 
 #define UART8250_FCR 0x02
 #define   UART8250_FCR_FIFO_EN		BIT(0) /* Fifo enable */