omap: add MMC and FAT support to SPL

- Add MMC raw and FAT mode boot support for OMAP
- Provide a means by which parameters passed by ROM-code
  can be saved in u-boot.
- Save boot mode related information passed by OMAP4 ROM-code
  and use it to determine where to load the u-boot from
- Assumes that the image has a mkimage header. Gets the
  payload size and load address from this header. If the
  header is not detected assume u-boot.bin as payload

Signed-off-by: Aneesh V <aneesh@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>