arm: make Arm GenericTimer a ClockedObject

arm: make Arm GenericTimer a ClockedObject

Within a device tree, the GenericTimer device needs to point (via phandle)
to a clock domain which is itself also an object in the device tree. Within
gem5, clock domains are managed by making all clocked SimObjects inherit
from ClockedObject rather than SimObject.

Without this change, the GenericTimer is unable to generate the appropriate
clock domain phandle, and will crash during DTB autogeneration.

Change-Id: I6d3fb6362847c6a01720b2f14b3d595d1e59f01f
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/4960
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>