The clk_xtal32k have clock-cells = <0>, drop the bogus specifier.

Fixes: 9509593f ("arm64: dts: imx8mm: Model PMIC to SNVS RTC clock path on Data Modul i.MX8M Mini eDM SBC")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>