arm64: dts: imx8mn-ddr4-evk: Describe the 32.768 kHz PMIC clock

The ROHM BD71847 PMIC has a 32.768 kHz clock.

Describe the PMIC clock to fix the following boot errors:

bd718xx-clk bd71847-clk.1.auto: No parent clk found
bd718xx-clk: probe of bd71847-clk.1.auto failed with error -22

Based on the same fix done for imx8mm-evk as per commit
a6a355ed ("arm64: dts: imx8mm-evk: Add 32.768 kHz clock to PMIC")

Fixes: 3e44dd09 ("arm64: dts: imx8mn-ddr4-evk: Add rohm,bd71847 PMIC support")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>