[POWERPC] Add powerpc/platforms/44x, disable platforms/4xx for now

This prepares for Ebony/440 support by creating an
arch/powerpc/platforms/44x directory.  It is populated with a single
misc_44x.S file, into which is moved the 44x specific reset code from
head_44x.S (on the grounds that we should really stop clogging up the
head_* files with random asm helper routines).

At the same time, we disable the (empty save Kconfig and Makefile)
arch/powerpc/platforms/4xx directory from the arch/powerpc/platforms
Makefile.  Contrary to the comment in
arch/powerpc/platforms/4xx/Makefile, attempting to build such an empty
Makefile will fail, thus breaking compile for the 44x platforms we're
about to add.  It can go back in once we start porting some of the 40x
platforms (and thus it becomes non-empty).

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