memory: atmel-ebi: Change naming scheme

The EBI block is not only available on at91 SoCs, but also on avr32 ones.
Change the structure and function prefixes from at91_ebi to atmel_ebi to
match this fact and make the prefix and driver name consistent.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>