ARM: at91: drop AT91_TIMER_HZ

Drop AT91_TIMER_HZ as this can be handled using HZ_FIXED. Initial help message
was:

On AT91rm9200 chips where you're using a system clock derived
from the 32768 Hz hardware clock, this tick rate should divide
it exactly: use a power-of-two value, such as 128 or 256, to
reduce timing errors caused by rounding.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>