ARM922T's icache isn't coherent with its dcache, so we need to ensure that its flushe...

ARM922T's icache isn't coherent with its dcache, so we need to ensure that its flushed before changing codecs.  Playback takes care of this for us, but only if we define cache coherency functions.  Do that.  Also, add a comment explaining that MMU functions need to be included so that other people don't spend hours chasing them in the future.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@21647 a1c6a512-1295-4272-9138-f99709370657