clocksource: Add Marvell Orion SoC timer

clocksource: Add Marvell Orion SoC timer

This patch add a DT enabled driver for timers found on Marvell Orion SoCs
(Kirkwood, Dove, Orion5x, and Discovery Innovation). It installs a free-
running clocksource on timer0 and a clockevent source on timer1.
Corresponding device tree documentation is also added.

Signed-off-by: Sebastian Hesselbarth <[email protected]>
Signed-off-by: Daniel Lezcano <[email protected]>
Tested-by: Ezequiel Garcia <[email protected]>
Tested-by: Andrew Lunn <[email protected]>