libpayload: timer: cast cpu_khz to make sure 64bit math is used

Change-Id: Iaf84de2330b433076a66c22fa72ffb45e957c0dc
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Found-by: Coverity Scan #1261177
Reviewed-on: https://review.coreboot.org/18034
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

payloads/libpayload/drivers/timer/rdtsc.c

@@ -37,7 +37,7 @@
 
 uint64_t timer_hz(void)
 {
-	return lib_sysinfo.cpu_khz * 1000;
+	return (uint64_t)lib_sysinfo.cpu_khz * 1000;
 }
 
 uint64_t timer_raw_value(void)