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/plat_nand.c b/drivers/mtd/nand/plat_nand.c
index 317aff4..caf5a73 100644
--- a/drivers/mtd/nand/plat_nand.c
+++ b/drivers/mtd/nand/plat_nand.c
@@ -95,7 +95,7 @@
 			goto out;
 	}
 
-	/* Scan to find existance of the device */
+	/* Scan to find existence of the device */
 	if (nand_scan(&data->mtd, pdata->chip.nr_chips)) {
 		err = -ENXIO;
 		goto out;