[POWERPC] Factor zImage's 44x reset code out of ebony.c

The ebony_exit() function which resets the Ebony board should in fact
be common to most if not all 44x boards.  This moves the function out
into 44x.c, renaming it, so it can be used by other 44x platforms.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>