staging/xgifb: fix addressing issues on platform with long physical addressing

Some platforms (e.g. ppc460ex) have 36-bit physical addressing, while
sizeof(unsigned long) == 4. Adapt xgifb driver to use phys_addr_t for
physical address variables instead of unsigned long.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>