ppc4xx/POST: Handle cached SDRAM correctly in Denali (440EPx) ECC POST

This patch fixes a problem in the Denali (440EPx) SDRAM ECC POST test.
When cache is enabled in the SDRAM area, the values written to SDRAM
need to be flushed from cache to SDRAM using the dcfb instruction.

Without this patch the POST ECC test failed. Now its working again on
platforms with cache enabled in SDRAM.

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