libpayload: timer: Revert timer_hz() return type to 64-bits

It seems that reducing the return type of timer_hz() to uint32_t in
CB:78888 was a bad idea... some Intel platforms actually use their raw
CPU clock for the timestamp counter which can be higher than 4GHz. This
patch reverts it back to uint64_t.

Also remove the redundant assertion in timer/generic.c since timer_us()
itself already does that check.

Cq-Depend: chromium:5274555
Change-Id: I471c7de7a28aec5bb965b23525ed579481ac8361
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80320
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Yidi Lin <yidilin@google.com>

payloads/libpayload/drivers/timer/rdtsc.c

@@ -35,10 +35,9 @@
 #include <arch/rdtsc.h>
 #include <assert.h>
 
-uint32_t timer_hz(void)
+uint64_t timer_hz(void)
 {
-	assert(UINT32_MAX / 1000 >= lib_sysinfo.cpu_khz);
-	return lib_sysinfo.cpu_khz * 1000;
+	return (uint64_t)lib_sysinfo.cpu_khz * 1000;
 }
 
 uint64_t timer_raw_value(void)