ARM: dts: imx7d-pico: Do not harcode the memory size

Currently the memory size described in dts is 2GB, which is incorrect.

There are 512MB and 1GB versions of imx7d-pico boards, so remove
the hardcoded memory size and let the bootloader pass the correct
value to the kernel.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>