splash: Load internal and external data from FIT

The FIT image could contain the splash data in 3 different structure:
- The splash data is embedded in FIT image (internal)
  In this case, the property 'data' presents in FIT image header. And
  internal information 'start' and 'end' represent the location and
  size of splash data inside of FIT image.
- The splash data is external with absolute position in FIT image
  This case is made by 'mkimage -p [pos]'. The splash data is stored
  at the absolute position. Instead the property 'data', the properties
  'data-position' and 'data-size' are used to specify the location and
  size of the splash data.
- the splash data is external with relative offset in FIT image
  This case is made by 'mkimage -E'. The splash data is placed after
  the FIT image header with 4 byte alignment. Instead the property
  'data', the properties 'data-offset' and 'data-size' are used to
  specify the location and size of the splash data.

Currently, the splash only support to load external data with relative
offset from FIT image. This commit make it possible to load the splash
data embedded in FIT image or the external data with absolute position

This inspiration comes from Simon Glass <sjg@chromium.org>, see
common/spl_fit.c

Signed-off-by: Leo Ruan <tingquan.ruan@cn.bosch.com>
Signed-off-by: Mark Jonas <mark.jonas@de.bosch.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefano Babic <sbabic@denx.de>