mx51: add support for pwm

This patch is adding support for pwm1 and pwm2 devices found
on mx51.
[ this patch has been tested with pwm-backlight driver ]

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>