[ARM] 4012/1: Clocksource for pxa

Add a clocksource driver for pxa2xx systems

Signed-off-by: Luotao Fu <lfu@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>