avr32: Use uncached() macro to get an address for SDRAM init

The paging system which is required to set up caching properties has not
yet been initialized when the SDRAM is initialized. So when the
map_physmem() function is converted to return the physical address
unchanged, the SDRAM initialization will break on some boards.

The avr32-specific uncached() macro will return an address which will
always cause uncached accessed to be made. Since this happens in the
board code, using avr32-specific features should be ok, and will allow
the SDRAM initialization to keep working.

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