arm64: dts: imx8mm-evk: correct pmic clock source

[ Upstream commit 85af7ffd ]

The osc_32k supports #clock-cells as 0, using an id is wrong, drop it.

Fixes: a6a355ed ("arm64: dts: imx8mm-evk: Add 32.768 kHz clock to PMIC")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>