zorro: Do not allocate zorro_autocon[] statically

Currently the array of Zorro devices is allocated statically, wasting
up to 4.5 KiB when running an Amiga or multi-platform kernel on a machine
with no or a handful of Zorro expansion cards. Convert it to conditional
dynamic memory allocation to fix this.

amiga_parse_bootinfo() still needs to store some information about the
detected Zorro devices, at a time even the bootmem allocator is not yet
available.  This is now handled using a much smaller array (typically less
than 0.5 KiB), which is __initdata and thus freed later.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>