[POWERPC] Make isa_mem_base common to 32 and 64 bits

This defines isa_mem_base on both 32 and 64 bits (it used to be 32 bits
only).  This avoids a few ifdef's in later patches and potentially can
allow support for VGA text mode on 64 bits powerpc.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>