mmc: atmel: Implement proper private data

Instead of passing just the register area as a private data, introduce
a proper struct atmel_mci_priv structure instead. This will become useful
in the subsequent patch, where we eliminate the global variable from this
driver.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
[fix free()]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>