avr32: data param to at32_add_device_mci() must be non-NULL

at32_add_device_mci() will refuse to add the mci device if the data
parameter is NULL. Fix up the favr-32 and hammerhead boards so that this
doesn't happen.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: Alex Raimondi <mailinglist@miromico.ch>
Cc: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>