ARM: dts: Drop bogus CLKSEL for timer12 on dra7

There is no CLKSEL for timer12 on dra7 unlike for timer1. This
causes issues on booting the device that Tomi noticed if
DEBUG_SLAB is enabled and the clkctrl clock does not properly
handle non-existing clock. Let's drop the bogus CLKSEL clock,
the clkctrl clock handling gets fixed separately.

Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>
Cc: Tero Kristo <t-kristo@ti.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Fixes: 4ed0dfe3 ("ARM: dts: dra7: Move l4 child devices to probe them with ti-sysc")
Signed-off-by: Tony Lindgren <tony@atomide.com>