libpayload: add *SECS_PER_SEC macros to stddef.h

this adds convenience definitions for MSECS_PER_SEC, USECS_PER_MSEC,
and USECS_PER_SEC along the lines of the time units in coreboot's
<timer.h>.

Change-Id: I489dc2d1ff55d137936acec74ac875dc7fbc1713
Signed-off-by: Caveh Jalali <caveh@google.com>
Reviewed-on: https://review.coreboot.org/20882
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/include/stddef.h

@@ -28,3 +28,7 @@ typedef __SIZE_TYPE__ ssize_t;
 #define KHz (1000)
 #define MHz (1000*KHz)
 #define GHz (1000*MHz)
+
+#define USECS_PER_SEC 1000000
+#define MSECS_PER_SEC 1000
+#define USECS_PER_MSEC (USECS_PER_SEC / MSECS_PER_SEC)