ARM: DTS: am57xx-beagle-x15: Select SYS_CLK2 for audio clocks

The tlv320aic3104 codec's master clock is coming from the SoC's CLKOUT2.
Select the SYS_CLK2 (via divider) as parent clock for CLKOUT2 and select
the same clock (SYS_CLK2) for McASP3 AHCLKX clock as well.
SYS_CLK2 is sourced from an external oscillator running 22.5792MHz and it
is coming in to the SoC via the X1_OSC1.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>