trivial: fix typos in comments s/DGBU/DBGU/

DBGU means Debug Unit, was refered as "DGBU" in some files. Fixed to "DBGU".

Signed-off-by: Samuel R. C. Vale <srcvale@holoscopio.com>
Acked-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

arch/arm/mach-at91/board-sam9g20ek.c

@@ -50,7 +50,7 @@ static void __init ek_map_io(void)
 	/* Initialize processor: 18.432 MHz crystal */
 	at91sam9260_initialize(18432000);
 
-	/* DGBU on ttyS0. (Rx & Tx only) */
+	/* DBGU on ttyS0. (Rx & Tx only) */
 	at91_register_uart(0, 0, 0);
 
 	/* USART0 on ttyS1. (Rx, Tx, CTS, RTS, DTR, DSR, DCD, RI) */