Fix references to the documentation files

Fix references to the documentation files

Many boot image configuration files refer to the
appropriate documentation file, but these references
contain typos in the directory and file name. Fix
them. Also fix reference to doc/README.SPL file.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Stefano Babic <sbabic@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>