[POWERPC] check firmware state before suspending

Currently the kernel blindly halts all the processors and calls the
ibm,suspend-me rtas call.  If the firmware is not in the correct
state, we then re-start all the processors and return.  It is much
smarter to first check the firmware state, and only if it is waiting,
call the ibm,suspend-me call.

Signed-off-by: Paul Mackerras <paulus@samba.org>