ppc4xx: Change ECC initialization on lwmon5 to use clean_dcache_range()

As it seems the "old" ECC initialization routine by using dflush() didn't
write all lines in the dcache back to memory on lwmon5. This could lead
to ECC error upon Linux booting. This patch changes the program_ecc()
routine to now use clean_dcache_range() instead of dflush().
clean_dcache_range() uses dcbst which is exactly what we want in this
case.

Since dflush() is known is cause problems, this routine will be
removed completely and replaced by clean_dcache_range() with an
additional patch.

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