mem: Optimize self-refresh entry

Self-refresh is entered during a refresh event, when the
rank was previously in a precharge power-down state.
The original code would enter self-refresh after a refresh
was issued.  The device subsequently will issue a refresh
on self-refresh entry.  On self-refresh exit, the controller
will issue another refresh command.

Devices require at least one additional refresh to be issued
between self-refresh exit and re-entry.  This ensures that enough
refreshes occur in the case when the device narrowly missed a
refresh on self-refresh exit.

To minimize the number of refresh operations and still maintain
the device requirement, the current logic does the following:
1) The controller will still enter self-refresh from a refresh
   event, when the previous state was precharge power-down.
   However, the refresh itself will be bypassed and the controller
   will immediately issue a self-refresh entry.
2) On a self-refresh exit, the controller will immediately
   issue a refresh command (per the original logic).  This ensures
   the devices requirements are met and is a convenient way to
   kick off the command state machine.

Change-Id: I1c4b0dcbfa3bdafd755f3ccd65e267fcd700c491
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10102
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

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