perf/imx_ddr: Add MODULE_DEVICE_TABLE

This is required for automatic probing when driver is built as a module.

Fixes: 9a66d36c ("drivers/perf: imx_ddr: Add DDR performance counter support to perf")
Acked-by: Frank Li <frank.li@nxp.com>
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Will Deacon <will@kernel.org>