hw/core/clock: introduce clock object

hw/core/clock: introduce clock object

This object may be used to represent a clock inside a clock tree.

A clock may be connected to another clock so that it receives update,
through a callback, whenever the source/parent clock is updated.

Although only the root clock of a clock tree controls the values
(represented as periods) of all clocks in tree, each clock holds
a local state containing the current value so that it can be fetched
independently. It will allows us to fullfill migration requirements
by migrating each clock independently of others.

This is based on the original work of Frederic Konrad.

Signed-off-by: Damien Hedde <damien.hedde@greensocs.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20200406135251.157596-2-damien.hedde@greensocs.com
[PMM: Use uint64_t rather than unsigned long long in trace events;
 the dtrace backend can't handle the latter]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>