powerpc/epapr: add "memory" as a clobber to all hypercalls

The "memory" clobber tells the compiler to ensure that all writes to memory
are committed before the hypercall is made.

"memory" is only necessary for hcalls where the Hypervisor will read or
write guest memory. However, we add it to all hcalls because the impact is
minimal, and we want to ensure that it's present for the hcalls that need
it.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>