arm64: dts: imx8mm-evk: Add 32.768 kHz clock to PMIC

The ROHM BD71847 PMIC has a 32.768 kHz clock.  Adding necessary parent
allows to probe the bd718x7 clock driver fixing boot errors:

    bd718xx-clk bd71847-clk.1.auto: No parent clk found
    bd718xx-clk: probe of bd71847-clk.1.auto failed with error -22
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-By: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mm-evk.dtsi

@@ -126,6 +126,10 @@ pmic@4b {
 		interrupts = <3 GPIO_ACTIVE_LOW>;
 		rohm,reset-snvs-powered;
 
+		#clock-cells = <0>;
+		clocks = <&osc_32k 0>;
+		clock-output-names = "clk-32k-out";
+
 		regulators {
 			buck1_reg: BUCK1 {
 				regulator-name = "buck1";