u-boot: align cache flushes in load_elf_image_shdr to line boundaries

u-boot: align cache flushes in load_elf_image_shdr to line boundaries

Prevent cache warning messages when using the 'bootelf' command on an
Arm target. Round down each section start address and round up the
respective section end to the nearest cache line.

Currently when using bootelf to load an image on Arm, several warnings
such as the following appear in the console:
  CACHE: Misaligned operation at range [87800000, 8783c5e0]
  CACHE: Misaligned operation at range [8783c5e0, 8784b3e0]

Signed-off-by: Neil Stainton <nstainton@asl-control.co.uk>
[trini: Reword commit message to include the info after the --- which
included the Signed-off-by line, and change ' at ' to '@']
Signed-off-by: Tom Rini <trini@konsulko.com>