powerpc/mpc85xx: work around erratum A-006593

Erratum A-006593 is "Atomic store may report failure but still allow
the store data to be visible".

The workaround is: "Set CoreNet Platform Cache register CPCHDBCR0 bit
21 to 1'b1.  This may have a small impact on synthetic write bandwidth
benchmarks but should have a negligible impact on real code."

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>