ARM: dts: bcm283x: Define memory at board level

Now with the varity of several RPi boards, the memory should be defined
at board level. This step gives us the chance to fix the memory size
of the RPi 1 B+, Zero (incl. W) and Compute Module 1.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Eric Anholt <eric@anholt.net>