ps3/block: Replace mtd/ps3vram by block/ps3vram

Convert the PS3 Video RAM Storage Driver from an MTD driver to a plain block
device driver.

The ps3vram driver exposes unused video RAM on the PS3 as a block device
suitable for storage or swap.  Fast data transfer is achieved using a local
cache in system RAM and DMA transfers via the GPU.

The new driver is ca. 50% faster for reading, and ca. 10% for writing.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>