[MIPS] ATA: Rename routerboard 500 to 532

The platform is actually named routerboard 532 so let's call it this.  This
patch only rename files, Kconfig and C symbols; no functional changes.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>