ARM: atmel: at91sam9x5ek: convert to generic board

Enable CONFIG_SYS_GENERIC_BOARD, so that we get rid of the following
warning on boot:

"Warning: Your board does not use generic board. Please read
doc/README.generic-board and take action. Boards not
upgraded by the late 2014 may break or be removed."

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