mediatek: Fix amount of memory on U7623

mediatek: Fix amount of memory on U7623

While finalizing support for the U7623 with 512MB, I made an embarresing
error and configured 1GB RAM for the board. I also forgot to move memory
from the dtsi and to the dts. This commit takes care of my mistakes.

While I am confessing my mistakes, I also note that I made a mistake in
the commit message of the initial U7623 commit. It is the .bin-file, and
not the .gz file that shall be sent to the device via tftp.

v1->v2:
* Remove redundant memory node (thanks Jonas Gorski)

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>