[MTD] [NAND] add s3c2440-specific read_buf/write_buf

Add read_buf/write_buf for s3c2440, which can read/write 32 bits at a
time rather than just 8. In my testing on an s3c2440a running at 400 MHz
with a 100 MHz HCLK, read performance improves by 36% (from 5.19 MB/s
to 7.07 MB/s).

Signed-off-by: Matt Reimer <mreimer@vpop.net>
Acked-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>