MTD: nand: make au1550nd.c a platform_driver

MTD: nand: make au1550nd.c a platform_driver

Transform the au1550nd.c driver into a platform_driver and hook it
up in the PB1550 board (gen_nand works fine on the DB1550, but since
I don't have a PB1550 to test this driver stays for now).

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Cc: linux-mtd@lists.infradead.org
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2875/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>