[POWERPC] powerpc: PA Semi PWRficient platform support

Base patch for PA6T and PA6T-1682M. This introduces the
arch/powerpc/platform/pasemi directory, together with basic
implementations for various setup.

Much of this was based on other platform code, i.e. Maple, etc.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>