target/ppc: Work [S]PURR implementation and add HV support

target/ppc: Work [S]PURR implementation and add HV support

The Processor Utilisation of Resources Register (PURR) and Scaled
Processor Utilisation of Resources Register (SPURR) provide an estimate
of the resources used by the thread, present on POWER7 and later
processors.

Currently the [S]PURR registers simply count at the rate of the
timebase.

Preserve this behaviour but rework the implementation to store an offset
like the timebase rather than doing the calculation manually. Also allow
hypervisor write access to the register along with the currently
available read access.

Signed-off-by: Suraj Jitindar Singh <[email protected]>
Reviewed-by: David Gibson <[email protected]>
[ clg: rebased on current ppc tree ]
Signed-off-by: Cédric Le Goater <[email protected]>
Message-Id: <20191128134700[email protected]>
Signed-off-by: David Gibson <[email protected]>