FIT: List kernel_noload in the list of types

In the source_file_format.txt file we talk about how to construct a
valid FIT image.  While it already says to look at the source for the
full list, add kernel_noload to the explicit list of types.  This is
arguably the most important type to use as most often we are including a
kernel that will run from wherever it is loaded into memory and execute.

This for example, allows you to create a single FIT image for Linux that
can be used on both OMAP and i.MX devices as the kernel will not need to
be moved in memory.

Signed-off-by: Tom Rini <trini@konsulko.com>