m68knommu: Use symbolic constants for cache operations on M54xx

Now that we have meaningfull symbolic constants for bit definitions
of the cache registers of m5407 and m548x chips, use them to
improve readability, portability and efficiency of the cache operations.

This also fixes __flush_cache_all for m548x chips : implicit
DCACHE_SIZE was exact for m5407, but wrong for m548x.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>