imx8qm_mek: Remove unused function detail_board_ddr_info()

detail_board_ddr_info() is not used anywhere, so simply remove it.

This function is only used by Layerscape, not by i.MX.

This was detected by the following sparse warning:

board/freescale/imx8qm_mek/imx8qm_mek.c:102:6: warning: no previous
prototype for ‘detail_board_ddr_info’ [-Wmissing-prototypes]