arm64: dts: imx8mn: Include 32kHz oscillator clock in SPL DTs

Since 50cdd3f74a ("clk: imx: clk-imx8mn Fix nand and spi clock parent"),
drivers/clk/imx/clk-imx8mn.c clk_get_by_name(dev, "osc_24m", &osc_24m_clk)
fails with error -22 EINVAL in SPL. This is because clk_get_by_name() in the
end calls fdtdec_parse_phandle_with_args(), which iterates over all phandles
in clock-controller@30380000 { clocks = <&osc_32k>, <&osc_24m>, ... } node
'clocks' property in an attempt to find the "osc_24m" clock, but fails to
resolve the &osc_32k phandle and returns with -EINVAL.

Include the osc_32k clock in SPL DTs as a low risk fix for v2025.01 release.
This way, fdtdec_parse_phandle_with_args() can resolve both the osc_32k and
following osc_24m phandle and successfully look up the osc_24m clock.

Fixes: 50cdd3f74a ("clk: imx: clk-imx8mn Fix nand and spi clock parent")
Signed-off-by: Marek Vasut <marex@denx.de>
Tested-by: Adam Ford <aford173@gmail.com> #imx8mn-beacon

arch/arm/dts/imx8mn-u-boot.dtsi

@@ -68,6 +68,11 @@
 	bootph-all;
 };
 
+&osc_32k {
+	bootph-pre-ram;
+	bootph-all;
+};
+
 #ifdef CONFIG_FSL_CAAM
 &sec_jr0 {
 	bootph-pre-ram;