[AVR32] Simplify early handling of memory regions

Use struct resource to specify both physical memory regions and
reserved regions and push everything into the same framework,
including kernel code/data and initrd memory. This allows us to get
rid of many special cases in the bootmem initialization and will also
make it easier to implement more robust handling of framebuffer
memory later.

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