Replaces occurrences of dm6443 with dm644x.

The cpu is decided based on the system_rev variable and it is being
compared with 6446 (Refer to arch/arm/mach-davinci/id.c for details).
So in the current tree, the function cpu_is_davinci_dm6443 will never
be true.
Signed-off-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>