ARM: shmobile: Specify device latencies for Mackerel devices directly

The results of adaptive latency computations in
GENPD_DEV_TIMED_CALLBACK() show that the start/stop and save/restore
state latencies of all devices on the Mackerel board I have tried are
a little below 250 us.  Therefore, if the 250 us is used as the
common initial value of the latency fields in struct gpd_timing_data
for all devices on Mackerel, the latency values will never have to
change at run time and there won't be any overhead related to
re-computation of the corresponding PM QoS data.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Magnus Damm <damm@opensource.se>