src/mainboard to src/security: Fix spelling errors

These issues were found and fixed by codespell, a useful tool for
finding spelling errors.

Signed-off-by: Martin Roth <martin@coreboot.org>
Change-Id: Ie34003a9fdfe9f3b1b8ec0789aeca8b9435c9c79
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58081
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/security/memory/memory_clear.c

@@ -98,7 +98,7 @@ static void clear_memory(void *unused)
 		__func__, (void *)pgtbl, (void *)vmem_addr);
 	}
 
-	/* Now clear all useable DRAM */
+	/* Now clear all usable DRAM */
 	memranges_each_entry(r, &mem) {
 		if (range_entry_tag(r) != BM_MEM_RAM)
 			continue;