gt64xxx: set isa_mem_base during registration

isa_mem_base is computed from registers during reset, but due to QEMU
limitations some devices (e.g. VGA card) need to know it earlier when
they are registered.

Workaround this by setting the value during registration instead of
reset.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
