boards: hifive1*: fix ticks per second

The HiFive1 boards power up using a 32 KiHz low frequency kernel, so
have a cycle rate of 32768 Hz.  The board definitions have not been
revisited since the Zephyr default for ticks-per-second increased from
100 to 10000.  The timer system on the board does not operate
correctly at 4 cycles per tick, but does at 328 cycles per tick.

To support functional timers while keeping system milliseconds in sync
with clock time set ticks-per-second to 128.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>