libpayload/drivers/timer: Use 64 bits to prevent overflow

Cast cpu_khz to a 64 bit integer to prevent possible
integer overflow (the multiplication is currently done
using 32 bit math). Similar to 61dac13 (libpayload:
timer: cast cpu_khz to make sure 64bit math is used).

Found-by: Coverity Scan, CID 1261177
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Change-Id: Iadb0abb7c7cc078f31a6d88d971f5d1b8ac62a9e
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32223
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

payloads/libpayload/drivers/timer/img_pistachio.c

@@ -18,7 +18,7 @@
 
 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)