cmd: fdt: Use start/size for chosen instead of start/end

cmd: fdt: Use start/size for chosen instead of start/end

Most U-Boot command deal with start/size instead of start/end. Convert
the "fdt chosen" command to use these semantics as well. The only user
of this subcommand is vexpress, so convert the smhload command to use
this as well. We don't bother renaming the variable in vexpress64's
bootcommand, since it will be rewritten in the next commit.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>