lib/ramtest: Add comment

Add a comment about the tested RAM region size.

Change-Id: I29e99a06777bd21a65aa67049ceede4fd8adb603
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/22199
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Maximilian Schander <coreboot@mimoja.de>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>

src/lib/ramtest.c

@@ -74,6 +74,8 @@ static inline void test_pattern(unsigned short int idx,
  * Simple write-read-verify memory test. See console debug output for
  * any dislocated bytes.
  *
+ * Tests 1MiB of memory starting from start.
+ *
  * @param start   System memory offset, aligned to 128bytes
  */
 static int ram_bitset_nodie(unsigned long start)