sparc,leon: Added support for AMBAPP bus.

The device is a AMBA bus if it is a child of prom node "ambapp" (AMBA
plug and play). Two functions
leon_trans_init() and leon_node_init() (defined in
sparc/kernel/leon_kernel.c) are called in the
prom_build_tree() path if CONFIG_SPARC_LEON is
defined. leon_node_init() will build up the device
tree using AMBA plug and play. Also: a extra check was addes to
prom_common.c:build_one_prop()
in case a rom-node is undefined which can happen for SPARC-LEON
because it creates only a minimum
nodes to emulate sparc behaviour.

Signed-off-by: Konrad Eisele <konrad@gaisler.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>