Split ISA and sysbus versions of m48t59 device

The m48t59 device supports both ISA and direct sysbus attached versions of
the device in the one .c file.  This can be awkward for some embedded
machine types which need the sysbus M48T59, but don't want to pull in the
ISA bus code and its other dependencies.

Therefore, this patch splits out the code for the ISA attached M48T59 into
its own C file.  It will be built when both CONFIG_M48T59 and
CONFIG_ISA_BUS are enabled.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>