clocksource: convert MXS timrotv2 to 32-bit down counting clocksource

Convert the MXS timrotv2 32-bit down counting clocksource to the
generic clocksource infrastructure.

Cc: Sascha Hauer <kernel@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>