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: Leonard Crestez <leonard.crestez@nxp.com>
Acked-by: Frank Li <frank.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>