avr32: data_bits should reflect the actual number of data bits

Make the data_bits enum in struct sdram_config reflect the actual number
of data bits on the bus, i.e. 16 or 32. Having 0 mean 16 bits and 1 mean
32 bits is just too confusing.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>