powerpc: Disable flush or invalidate dcache by range for some SoCs

Commit ac337168 unified functions to flush and invalidate dcache by
range. These two functions were no-op for SoCs other than 4xx and
MPC86xx. Adding these functions seemed to be correct but introduced
issues when the dcache is flushed. While the root cause is under
investigation, disable these functions for affected SoCs so various
drivers can work.

Signed-off-by: York Sun <york.sun@nxp.com>