m68k/mac: Move struct mac_booter_data to <asm/macintosh.h>

struct mac_booter_data is no longer part of the bootinfo API, hence move it
from <asm/bootinfo.h> to <asm/macintosh.h>, dropping all unused fields in
the process.

Also remove the no longer used mac_booter_data pointer from head.S.

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