Update ffff4000.s

writeout/clarify all of 'start'.
Add more details to the BROM's boot function

The boot function loops over the various possible boot media (MMC0,
NAND, SPINOR, MMC3) and tries to load the SPL. This patch adds comments
about each step of the 'boot' function.

Signed-off-by: Oliver Schinagl <oliver@schinagl.nl>