dev-arm: Fix checkpointing for the GenericTimer

dev-arm: Fix checkpointing for the GenericTimer

The revamp of the GenericTimer was not taking into account:

* The name of the variable will be printed on the checkpoint to label the
data. It is not possible to use different variable names when
serializing/unserializing, and it is not possible to use the same
temporary variable to serialize/unserialize different values.

* the serializeSection is creating a new sub section in the
checkpoint. Doing the following:

void
GenericTimerFrame::serialize(CheckpointOut &cp) const
{
    physTimer.serializeSection(cp, "phys_timer");
    virtTimer.serializeSection(cp, "virt_timer");
    SERIALIZE_SCALAR(accessBits);
}

will serialize the accessBits under the virt_timer subsection
rather than the parent generic_timer_frame.

JIRA: https://gem5.atlassian.net/projects/GEM5/issues/GEM5-426

Change-Id: I7676309965a33156789d2ef13e966c7a4ad88a71
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27708
Tested-by: kokoro <noreply+kokoro@google.com>