arm64: defconfig: Build imx8 ddr pmu as module

This is available on all imx8 but is not "boot critical" in any way so
build as a module.
Signed-off-by: NLeonard Crestez <leonard.crestez@nxp.com>
Acked-by: NFrank Li <frank.li@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

arch/arm64/configs/defconfig

@@ -785,6 +785,7 @@ CONFIG_PHY_ROCKCHIP_TYPEC=y
 CONFIG_PHY_UNIPHIER_USB2=y
 CONFIG_PHY_UNIPHIER_USB3=y
 CONFIG_PHY_TEGRA_XUSB=y
+CONFIG_FSL_IMX8_DDR_PMU=m
 CONFIG_HISI_PMU=y
 CONFIG_QCOM_L2_PMU=y
 CONFIG_QCOM_L3_PMU=y