The amount of available memory is clearly a board-specific value, so
the core per-soc dtsi should not define a default of any sort.
Therefore move the memory-nodes to the two board files.

Also fix the amount of memory on Kylin (512MB instead of 1GB).
While in most cases the bootloader will override this with the
actual amount of memory, there is no need to keep known wrong values
in the board-dts.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>