stddef: Add KHz, MHz and GHz constants

This patch adds some simple constants to more easily write and do math
with frequencies, analogous to the existing KiB, MiB and GiB constants
for sizes.

BUG=None
TEST=Compiled Veyron_Pinky.

Original-Change-Id: I4a1927fd423eb96d3f76f7e44b451192038b02e0
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/221800
Original-Reviewed-by: David Hendricks <dhendrix@chromium.org>

(cherry picked from commit 41bb802681)
Signed-off-by: Aaron Durbin <adurbin@chromium.org>

Change-Id: I1e708b0aa53533c9ab999793ca2273c6dc68b5f6
Reviewed-on: http://review.coreboot.org/9253
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/include/stddef.h

@@ -26,6 +26,10 @@ typedef unsigned int wint_t;
 /* Could we ever run into this one? I hope we get this much memory! */
 #define TiB (1<<40)
 
+#define KHz (1000)
+#define MHz (1000 * KHz)
+#define GHz (1000 * MHz)
+
 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
 
 #define check_member(structure, member, offset) _Static_assert( \