libpayload: improve us timer accuracy

In cases where timer clock frequency is not an integer number of
megahertz, the calculations in timer_us() lack accuracy.

This patch modifies calculations to reduce the error. The maximum
interval this calculation would support decreases, but it still is in
excess of 1844674 seconds for a timer clocked by 10 MHz, which is more
than enough.

BUG=none
TEST=manual
  . verified timer accuracy using a depthcharge CLI command

Original-Change-Id: Iffb323db10e74b0ce3b4d59a56983bfee12e6805
Original-Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/207358
Original-Reviewed-by: David Hendricks <dhendrix@chromium.org>
(cherry picked from commit e1abf87d438de1a04714482d5b610671e8cc0663)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>

Change-Id: Ia892726187ab040dd235f493c92856c15951cc06
Reviewed-on: http://review.coreboot.org/8128
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)

payloads/libpayload/libc/time.c

@@ -204,5 +204,5 @@ u64 timer_us(u64 base)
 		}
 	}
 
-	return timer_raw_value() / (hz / 1000000) - base;
+	return (1000000 * timer_raw_value()) / hz - base;
 }