lwmon5 watchdog: limit trigger rate

lwmon5 watchdog: limit trigger rate

Limit the rate of h/w watch-dog triggering on the LWMON5 board by
the CONFIG_WD_MAX_RATE value.

Note that an earlier version of this patch which used microseconds
instead of ticks dis not work. The problem was that we used
usec2ticks() to convert microseconds into ticks. usec2ticks() uses
get_tbclk(), which in turn calls get_sys_info(). It turns out that
this function does a lot of prolonged operations (like divisions)
which take too much time so we do not trigger the watchdog in time,
and it resets the system.

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>