boards: google_dragonclaw: limit frequency of an unused clock

I2S is unused on the dragonclow board. Increase the R division factor
(used for I2S), to reduce the clock frequency, which saves some power.

Signed-off-by: Dawid Niedzwiecki <[email protected]>

boards/arm/google_dragonclaw/google_dragonclaw.dts

@@ -37,6 +37,7 @@
 	mul-n = <192>; /* 16MHz * 192/8 = 384MHz VCO clock */
 	div-p = <4>; /* 96MHz PLL general clock output */
 	div-q = <8>; /* 48MHz PLL output for USB, SDIO, RNG */
+	div-r = <7>; /* I2S - lowest possible frequency to save power */
 	clocks = <&clk_hsi>;
 	status = "okay";
 };