[AVR32] Get rid of board_setup_fbmem()

Since the core setup code takes care of both allocation and
reservation of framebuffer memory, there's no need for this board-
specific hook anymore. Replace it with two global variables,
fbmem_start and fbmem_size, which can be used directly.

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