ARM: dts: armada-xp: enable buffer manager support on Armada XP boards

Since mvneta driver supports using hardware buffer management (BM), in
order to use it, board files have to be adjusted accordingly. This commit
enables BM on AXP-DB and AXP-GP in same manner - because number of ports
on those boards is the same as number of possible pools, each port is
supposed to use single pool for all kind of packets.

Moreover appropriate entry is added to 'soc' node ranges, as well as "okay"
status for 'bm' and 'bm-bppi' (internal SRAM) nodes.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>