ARM: at91: fix and update README.at91 document

This patch implement following things
  - The link no longer accessable
  - Remove the error configuration command
  - Update soldered data flash memory map
  - Update at91sam9m10g45ek memory size to 128MiB

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