The e1000 driver uses the dma coherent functions
and thus can only be build for architectures implementing them.

Here is mips malta build error log:

  drivers/built-in.o: In function `e1000_transmit':
  drivers/net/e1000.c:(.text.e1000_transmit+0x80): undefined reference to
  `dma_sync_single_for_device'
  drivers/built-in.o: In function `e1000_poll':
  drivers/net/e1000.c:(.text.e1000_poll+0x60): undefined reference to
  `dma_sync_single_for_cpu'

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>