hw/net: add DEC Tulip NIC emulation

This patch adds emulation of DEC/Intel Tulip 21143 with some external chips:
  * Intel LXT971A 10/100 Mbps PHY MII Tranceiver;
  * Microchip 93LC46B 1K Microwire Compatible Serial EEPROM.

Restrictions and TODOs:
  - Tulip always work in promisc-mode with no packet filtering
    (TODO: check qemu packet filtering interfaces)

  - Address errors in packet descriptors are not checked

  - Tulip is bound to PCI, no other bus is possible for now
    (TODO: Rewrite code to permit Tulip to live on buses other than PCI)

  - Internal transceiver is not emulated
    The reason: internal transceiver works with 10 Mbps but we need 100Mbps
    which only external transceivers can give.

  - Link status is not emulated, it's always ON
    (TODO: use standard qemu interface)

  - Incorrect behavior of the Tulip driver is not checked and there's no error
    signaling via register CSR5

  - Only transfer/receive (TI/RI) interrupts are emulated

  - Subsystem IDs in EEPROM image are not good
    (TODO: find proper IDs)

TESTING
-------

NOTE:
  buildroot with tulip-friendly config for qemu is here:
    https://github.com/dmitrysmagin/buildroot.git : master
    (configs/qemu_mips_malta_tulip_defconfig)

  $ git clone https://github.com/dmitrysmagin/qemu.git
  $ cd qemu
  $ ./configure --target-list=mips-softmmu
  ...
  $ make
  ...
  $ ./mips-softmmu/qemu-system-mips \
       -M malta \
       -m 256 \
       -nodefaults \
       -nographic \
       -kernel qemu-malta-testing/vmlinux \
       -initrd qemu-malta-testing/rootfs.cpio \
       -serial stdio \
       -net nic,vlan=0,model=tulip,macaddr=00:11:22:33:44:ee

  Linux version 3.12.0 (exmortis@lazar) (gcc version 4.7.3 (Sourcery CodeBench Lite 2013.05-66) ) #1 SMP Thu Nov 14 11:58:04 MSK 2013
  ...
  Welcome to Buildroot
  buildroot login: root
  # lspci -d 0x1011:
  00:12.0 Ethernet controller: Digital Equipment Corporation DECchip 21142/43 (rev 41)
  # ifconfig eth0
  eth0      Link encap:Ethernet  HWaddr 00:11:22:33:44:EE
            BROADCAST MULTICAST  MTU:1500  Metric:1
            RX packets:0 errors:0 dropped:0 overruns:0 frame:0
            TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
            collisions:0 txqueuelen:1000
            RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

  To test data transfer please use '-net tap,vlan=0,...' or
  '-net vde,vlan=0,...' (depends on your local network settings).

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Dmitry Smagin <exmortis@yandex.ru>
QEMU-MALTA-TESTING: Add linux kernel 3.12 and rootfs.cpio