ARM: dts: at91: sama5d2_xplained: remove wrong memory node

The size field of the memory node is wrong. Rely on the default value in
sama5d2.dtsi that happens to be correct for the SAMA5D2 Xplained board.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>