avr32: allow multi block mmc access for all boards

avr32: allow multi block mmc access for all boards

Commit 1db7377a70a8d931c32648e717695133120d5456 fixes the gen_atmel_mci driver
to be able to use multi block access for avr32. Therefore remove the setting
which forces single block access.

This also adds a huge performace gain for mmc access:
---8<---
Loading file "/boot/uImage" from mmc device 0:1
1830666 bytes read in 1293 ms (1.3 MiB/s)
--->8---

vs.

---8<---
Loading file "/boot/uImage" from mmc device 0:1
1830666 bytes read in 237 ms (7.4 MiB/s)
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: haavard.skinnemoen@atmel.com
Cc: hans-christian.egtvedt@atmel.com
Cc: mpfj@mimc.co.uk
Cc: alex.raimondi@miromico.ch
Cc: julien.may@miromico.ch
Cc: egtvedt@samfundet.no
Cc: havard@skinnemoen.net