ARM: dts: imx6ul-pico-hobbit: Do not hardcode the memory size

There are two variants of imx6ul-pico boards: one with 256MB and
another one with 512MB of RAM.

Do not hardcode the memory size in the device tree and let the
bootloader fill the correct value instead.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>