arm64: mpam: Context switch the MPAM registers

MPAM has a system register that is used to hold the partid and pmg values
that traffic generated by EL0 will use. This can be set per-task by the
resctrl file system.

Add a helper to switch this. resctrl expects a 'default' value to be
used in preference if the default partid and pmg are selected.

struct task_struct's separate closid and rmid fields are insufficient
to implement resctrl using MPAM, as resctrl can change the partid (closid)
and pmg (sort of like the rmid) separately. On x86, the rmid is an
independent number, so a race that writes a mismatched  closid and rmid
into hardware is benign. On arm64, the pmg bits extend the partid.
(i.e. partid-5 has a pmg-0 that is not the same as partid-6's pmg-0).
In this case, mismatching the values will 'dirty' a pmg value that
resctrl believes is clean, and is not tracking with its 'limbo' code.

To avoid this, the partid and pmg are always read and written as a pair.
Instead of making struct task_struct's closid and rmid fields an
endian-unsafe union, add the value to struct thread_info and always use
READ_ONCE()/WRITE_ONCE() when accessing this field.

Resctrl (the defacto interface for controlling this stuff) allows a
per-cpu 'default' value to be set, this overrides the values when
scheduling a task in the default control-group, which has PARTID 0.

The current system register value is kept in a per-cpu variable to
avoid writing to the system register if the value isn't going to change.
Writes to this register may reset the hardware state for regulating
bandwidth.

Finally, there is no reason to context switch these registers unless
there is a driver changing the values in struct task_struct. Hide
the whole thing behind a static key. This also allows the driver to
disable MPAM in response to errors reported by hardware.

By the same logic, add a Kconfig hook to disable context switch on
kernels that will never see MPAM. The driver will depend on this
Kconfig symbol too.

CC: Amit Singh Tomar <amitsinght@marvell.com>
Signed-off-by: James Morse <james.morse@arm.com>