mmc: atmel-mci: use endian agnostic IO

Change the __raw IO functions to endian agnostic relaxed ones to allow
the driver to function on big endian ARM systems.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>