mtd, spi: Add MTD layer driver

mtd, spi: Add MTD layer driver

Add MTD layer driver for spi, original patch from:
http://git.denx.de/?p=u-boot/u-boot-mips.git;a=commitdiff;h=bb246819cdc90493dd7089eaa51b9e639765cced

Changes from Heiko Schocher against this patch:
- Remove compile error if not defining CONFIG_SPI_FLASH_MTD:

  LD      drivers/mtd/spi/built-in.o
drivers/mtd/spi/sf_probe.o: In function `spi_flash_mtd_unregister':
/home/hs/abb/imx6/u-boot/drivers/mtd/spi/sf_internal.h:168: multiple definition of `spi_flash_mtd_unregister'
drivers/mtd/spi/sf_params.o:/home/hs/abb/imx6/u-boot/drivers/mtd/spi/sf_internal.h:168: first defined here
drivers/mtd/spi/sf_ops.o: In function `spi_flash_mtd_unregister':
/home/hs/abb/imx6/u-boot/drivers/mtd/spi/sf_internal.h:168: multiple definition of `spi_flash_mtd_unregister'
drivers/mtd/spi/sf_params.o:/home/hs/abb/imx6/u-boot/drivers/mtd/spi/sf_internal.h:168: first defined here
make[1]: *** [drivers/mtd/spi/built-in.o] Fehler 1
make: *** [drivers/mtd/spi] Fehler 2

- Add a README entry.
- Add correct writebufsize, to fit with Linux v3.14
  MTD, UBI/UBIFS sync.

Note (From Jagan): For testing raw mtd parition erase/read/write operations
using cmd_sf, sf_mtd should be required to register the spi flash device to
MTD layer but the sf_mtd_info ops were not required until and unless if we
use any flash filesystem layer say for example UBI. Due to this the foot-print
got increased ~290bytes in non-UBI case here that should be acceptible.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Heiko Schocher <hs@denx.de>
Tested-by: Jagannadh Teki <jteki@openedev.com>
Reviewed-by: Jagannadh Teki <jteki@openedev.com>