ppc4xx: Flush complete dcache in relocate_code()

When the cache is enabled in SDRAM we need to flush not only the global
data area but also the bd_info struct in relocate_code. This patch now
flushed the complete dcache (all dcache lines) via flush_dcache() instead
of adding a flush_dcache_range() call for bd_info since this is faster.

Signed-off-by: Stefan Roese <sr@denx.de>