mvebu: use SOC to derive DEVICE_DTS

mvebu: use SOC to derive DEVICE_DTS

This introduces the SOC variable to mvebu target to derive some of
the DEVICE_DTS variables based on the SOC prefix and the device
definition name.

Since DTS names and compatible are inconsistent also in the kernel
for this target, the scheme cannot be applied to all devices, though.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Acked-by: Tomasz Maciej Nowak <tomek_n@o2.pl>