[POWERPC] Update smu command definitions

This updates smu.h with several new commands, and adds parameter
descriptions for existing commands.

Signed-off-by: Michael Hanselmann <linux-kernel@hansmi.ch>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>