sparc: Use image_setup_linux() instead of local code

Sparc only really sets up the ramdisk, but we should still use
image_setup_linux() so that setup is common across all architectures
that use the FDT.

Cover-letter
Introduce a common image_setup_linux() function
This series continues the work to tidy up the image code. Each
architecture has its own code for setting up ready for booting linux.
An attempt is made here to unify these in a single image_setup_linux()
function.

The part of the image code that deals with FDT is split into image-fdt.c
and a few tweaks are added to make FIT images more viable in SPL.
END
Signed-off-by: Simon Glass <sjg@chromium.org>