ARM: at91: PIT: Move the driver to drivers/clocksource

Now that we don't depend on anyting in the mach-at91 directory, we can just
move the driver to where it belongs.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Conflicts:
	arch/arm/mach-at91/Kconfig
	arch/arm/mach-at91/Makefile