mpc512x: Move common files to share them by several boards

We will soon see several new MPC521x based boards added.  This patch
moves files that are not board specific to a common directory so they
can be shared by all such ports.  It also splits off common IDE code
into a new file, cpu/mpc512x/ide.c .

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: John Rigby <jcrigby@gmail.com>