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/libc/time.c

@@ -171,14 +171,15 @@ void arch_ndelay(uint64_t ns)
 
 u64 timer_us(u64 base)
 {
-	static u32 hz, mult = USECS_PER_SEC;
+	static u64 hz;
+	static u32 mult = USECS_PER_SEC;
 	u32 div;
 
 	// Only check timer_hz once. Assume it doesn't change.
 	if (hz == 0) {
 		hz = timer_hz();
 		if (hz < mult) {
-			printf("Timer frequency %" PRIu32 " is too low, "
+			printf("Timer frequency %" PRIu64 " is too low, "
 			       "must be at least 1MHz.\n", hz);
 			halt();
 		}