clocksource: Add J-Core timer/clocksource driver

clocksource: Add J-Core timer/clocksource driver

At the hardware level, the J-Core PIT is integrated with the interrupt
controller, but it is represented as its own device and has an
independent programming interface. It provides a 12-bit countdown
timer, which is not presently used, and a periodic timer. The interval
length for the latter is programmable via a 32-bit throttle register
whose units are determined by a bus-period register. The periodic
timer is used to implement both periodic and oneshot clock event
modes; in oneshot mode the interrupt handler simply disables the timer
as soon as it fires.

Despite its device tree node representing an interrupt for the PIT,
the actual irq generated is programmable, not hard-wired. The driver
is responsible for programming the PIT to generate the hardware irq
number that the DT assigns to it.

On SMP configurations, J-Core provides cpu-local instances of the PIT;
no broadcast timer is needed. This driver supports the creation of the
necessary per-cpu clock_event_device instances.

A nanosecond-resolution clocksource is provided using the J-Core "RTC"
registers, which give a 64-bit seconds count and 32-bit nanoseconds
that wrap every second. The driver converts these to a full-range
32-bit nanoseconds count.

Signed-off-by: Rich Felker <dalias@libc.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Cc: linux-sh@vger.kernel.org
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Link: http://lkml.kernel.org/r/b591ff12cc5ebf63d1edc98da26046f95a233814.1476393790.git.dalias@libc.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>