ARM: atmel: cleanup: remove at91cap9 related code

As the at91cap9adk board is removed by commit: b5508344
(ARM: remove broken "at91cap9adk" board), so the at91cap9
code is not used anymore, and also the document for
at91cap9 can not be found on www.atmel.com, so remove the
at91cap9 related code.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Acked-by: Andreas Bießmann <andreas.devel@googlemail.com>