Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

These files will support the whole at91sam9x5 family when done,
so rename them now before they get copied further afield...

Eliminate the AT91XXXX_BASE for each SoC. AT91_BASE is the right way
to spell this since we only have one AT91_BASE for all Atmel arm9 SoCs.

Add PIOD, make at91sam9x25 a standard SoC, tweak some comments.

Initital support for AT91SAM9X25 SoC and the SAM9X25-EK evaluation
board. Much work remains.