clocksource/drivers/nomadik-mtu: Handle 32kHz clock

It happens on the U8420-sysclk Ux500 PRCMU firmware
variant that the MTU clock is just 32768 Hz, and in this
mode the minimum ticks is 5 rather than two.

I think this is simply so that there is enough time
for the register write to propagate through the
interconnect to the registers.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200628220153.67011-1-linus.walleij@linaro.org