[POWERPC] Add export of vgacon_remap_base

The following macro :
include/asm-powerpc/vga.h:#define VGA_MAP_MEM(x,s) (x + vgacon_remap_base)
is used by drivers/video/console/vgacon.c which can be compiled as a module
(drivers/video/vga16fb.ko).

Therefore, vgacon_remap_base should be exported.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>