IXP4xx: change the timer base frequency to 66.666000 MHz.

Clock generators used by IXP4xx processors are usually 33.333 MHz, sometimes
33.33 MHz and few platforms use 33 MHz. The timers tick twice as fast,
that means 66.666, 66.66 or 66 MHz. Current 66.666666 MHz means 10 ppm
offset from the usual 66.666 MHz.

Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>