hw/openrisc: Initialize timer time at startup

hw/openrisc: Initialize timer time at startup

The last_clk time was initialized at zero, this means when we calculate
the first delta we will calculate 0 vs current time which could cause
unnecessary hops.

This patch moves timer initialization to the cpu reset.  There are two
resets registered here:

 1. Per cpu timer mask (ttmr) reset.
 2. Global cpu timer (last_clk and ttcr) reset, attached to the first
    cpu only.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>