Add a device driver for the Altera University Program SD Card IP Core,

which can be synthesised in Altera FPGAs.  An altera_sdcardc device
probes during the boot, and /dev/altera_sdcard devices come and go as
inserted and removed.  The device driver attaches directly to the
Nexus, as is common for system-on-chip device drivers.

This IP core suffers a number of significant limitations, including a
lack of interrupt-driven I/O -- we must implement timer-driven polling,
only CSD 0 cards (up to 2G) are supported, there are serious memory
access issues that require the driver to verify writes to memory-mapped
buffers, undocumented alignment requirements, and erroneous error
returns.  The driver must therefore work quite hard, despite a fairly
simple hardware-software interface.  The IP core also supports at most
one outstanding I/O at a time, so is not a speed demon.

However, with the above workarounds, and subject to performance
problems, it works quite reliably in practice, and we can use it for
read-write mounts of root file systems, etc.

Sponsored by:	DARPA, AFRL

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