clocksource: vf_pit_timer: use complement for sched_clock reading

Vybrids PIT register is monitonic decreasing. However, sched_clock
reading needs to be monitonic increasing. Use bitwise not to get
the complement of the clock register. This fixes the clock going
backward. Also, the clock now starts at 0 since we load the
register with the maximum value at start.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Cc: daniel.lezcano@linaro.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux@arm.linux.org.uk
Link: http://lkml.kernel.org/r/d25af915993aec1b486be653eb86f748ddef54fe.1394057313.git.stefan@agner.ch


Cc: stable@vger.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>