Add a cast to a logging printf to avoid a compilation failure
if target_phys_addr_t is a 64 bit type. (This is better than
using TARGET_FMT_plx because we really don't need a full
16 digit hex string to print the offset into a device.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>