ARM: dts: imx6: use correct mclk frequency for audio codec

The reference clock for the SGTL5000 is generated by a 26MHz crystal
oscillator on the Ka-Ro electronics STK5 eval kits. Use the correct
frequency setting in DTB.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>