ARM: dts: Use 32KiHz oscillator on devkit8000

Devkit8000 board seems to always used 32k_counter as clocksource.
Restore this behavior.

If clocksource is back to 32k_counter, timer12 is now the clockevent
source (as before) and timer2 is not longer needed here.

This commit fixes the same issue observed with commit 23885389
("ARM: dts: Fix timer regression for beagleboard revision c") when sleep
is blocked until hitting keys over serial console.

Fixes: aba1ad05 ("clocksource/drivers/timer-ti-dm: Add clockevent and clocksource support")
Fixes: e428e250 ("ARM: dts: Configure system timers for omap3")
Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>