BCM2708: DT: change 'axi' nodename to 'soc'

Change DT node named 'axi' to 'soc' so it matches ARCH_BCM2835.
The VC4 bootloader fills in certain properties in the 'axi' subtree,
but since this is part of an upstreaming effort, the name is changed.

Signed-off-by: Noralf Tronnes <notro@tronnes.org>