Add hooks for plugging platform-provided transfer backend.

In order to use platorm backend hardware driver should
impement three methods:
- platform_start_transfer and platform_finish_transfer
    to start and finish transfer
- platform_will_handle - check whether transaction is
    suitable for backend. If not - driver will fall back
    to PIO mode.

Submitted by:	Daisuke Aoyama <aoyama at peach.ne.jp>
Approved by:	ian@
Add driver for BCM2835's DMA engine

This is a version of code submitted by Daisuke Aoyama <aoyama at peach.ne.jp>
with some architectural changes.
Add platform DMA support to SDHCI driver for BCM2835

Submitted by:	Daisuke Aoyama <aoyama at peach.ne.jp>
Reviewed by:	ian@
Change the ixgbe module name to if_ixgbe to conform
to the usual naming convention.