sh: Fix OF_SEPARATE support

If the OF_SEPARATE is enabled, the DT is appended past the _end symbol.
The current code however clears BSS very early, which overwrites the DT
blob with zeroes. Moreover, the early code relocates U-Boot into RAM to
the correct location, but does not relocate the DT.

This patch adds code to relocate the DT and avoids clearing BSS too
early, thus addressing both problems with OF_SEPARATE on SH.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>