arm: imx8m: imx8mm-cl-iot-gate: Add support for detect memory size

arm: imx8m: imx8mm-cl-iot-gate: Add support for detect memory size

When purchasing imx8mm-cl-iot-gate it is able to customize the
memory size. It could be 1GB, 2GB and 4GB. We implement
board_phys_sdram_size() to detect the memory size for usage.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Frieder Schrempf <frieder.schrempf@kontron.de>
Cc: uboot-imx <uboot-imx@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>