sparc32_dma: remove is_ledma hack and replace with memory region alias

This hack originated from before the memory region API was introduced, and
increased the size of the ledma DMA device to capture incorrect accesses
beyond the end of the ledma device. A full analysis can be found on Artyom's
blog at http://tyom.blogspot.co.uk/2010/10/bug-in-all-solaris-versions-after-57.html.

With the memory API we can now simply alias the incorrect access onto its
intended destination allowing us to remove the hack.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Artyom Tarasenko <atar4qemu@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
