ARM: at91/dt: fix macb node declaration

Macb0 node cannot be activated in generic sam9x5ek.dtsi file
as the sam9g15 does not have one.
Move the macb0 & macb1 activation in board .dts file that
support them.

Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>