fbdev/cirrusfb: Add support for Picasso IV in Zorro II mode

In Zorro II mode, the Village Tronic Picasso IV graphics card shows up as
either 2 or 3 Zorro devices:
  - One for the registers of the Cirrus Logic graphics chip,
  - One for the first bank of 2 MiB of graphics memory,
  - An optional one for the second bank of 2 MiB of graphics memory,
    if there was enough free Zorro II address space.

Based on src/sys/arch/amiga/dev/grf_cl.c from NetBSD.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>