powerpc/pseries/hibernation: switch to rtas_ibm_suspend_me()

rtas_suspend_last_cpu() and related code perform a lot of work that
isn't relevant to the hibernation workflow. All other CPUs are offline
when called so there is no need to place them in H_JOIN or prod them
on resume, nor is there need for retries or operations on shared
state.

Call the rtas_ibm_suspend_me() wrapper function directly from
pseries_suspend_enter() instead of using rtas_suspend_last_cpu().

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201207215200.1785968-23-nathanl@linux.ibm.com