Since all currently supported at91rm9200 boards are migrated to at91
support the joining notice can be removed.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>

* prepare joining at91 and at91rm9200
 * add modified copy of soc files to cpu/arm920t/at91 to make
   possible to compile at91rm9200 boards in at91 tree instead
   of at91rm9200
 * add header files with c structure defs for AT91 MC, ST and TC
 * the new cpu files are using at91 c structure soc access
 * please read README.soc-at91 for details

Signed-off-by: Jens Scharsig <js_at_ng@scharsoft.de>

* add's the new temporary CONFIG_AT91_LEGACY to all board configs
 This will need for backward compatiblity, while change the SoC access
 to c structures. If CONFIG_AT91_LEGACY is defined, the deprecated
 SoC is used.

Signed-off-by: Jens Scharsig <js_at_ng@scharsoft.de>