ARM: dts: am335x-evmsk: Correct audio clock frequency

The clock for audio is sourced from virt_24000000_ck, so the correct
frequency is 24000000.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
CC: <stable@vger.kernel.org> # 3.13.x
Signed-off-by: Tony Lindgren <tony@atomide.com>