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)

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Dmitry Smagin <exmortis@yandex.ru>