Fix common misspellings

Fixes generated by 'codespell' and manually reviewed.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
diff --git a/drivers/mtd/nand/autcpu12.c b/drivers/mtd/nand/autcpu12.c
index 7c95da1..0911cf0 100644
--- a/drivers/mtd/nand/autcpu12.c
+++ b/drivers/mtd/nand/autcpu12.c
@@ -176,7 +176,7 @@
 	 */
 	this->options = NAND_USE_FLASH_BBT;
 
-	/* Scan to find existance of the device */
+	/* Scan to find existence of the device */
 	if (nand_scan(autcpu12_mtd, 1)) {
 		err = -ENXIO;
 		goto out_ior;