overo: Split overo.c into spl.c, common.c and overo.c

This separates the SPL-specific code from the u-boot-specific code for
the Overo board following the discussion at
http://lists.denx.de/pipermail/u-boot/2015-April/211622.html



The code is split up into spl.c, overo.c and common.c (which
has the code common to both)

Signed-off-by: Arun Bharadwaj <arun@gumstix.com>