avr32: Rework SDRAM initialization code

This cleans up the SDRAM initialization and related code a bit, and
allows faster booting.

  * Add definitions for EBI and internal SRAM to asm/arch/memory-map.h
  * Remove memory test from sdram_init() and make caller responsible
    for verifying the SDRAM and determining its size.
  * Remove base_address member from struct sdram_config (was sdram_info)
  * Add data_bits member to struct sdram_config and kill CFG_SDRAM_16BIT
  * Add support for a common STK1000 hack: 16MB SDRAM instead of 8.

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