Documentation: Add powerpc options for spec_store_bypass_disable

Document the support for spec_store_bypass_disable that was added for
powerpc in commit a048a07d ("powerpc/64s: Add support for a store
forwarding barrier at kernel entry/exit").

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>