mx5: change usb clock source from pll3 to pll2

For power management reasons, pll2 should be used to source the USBOH3
clock for mx51. PLL3 can be completely gated off when USB is not used.

This patch applies to 2.6.34-rc7.

Signed-off-by: Dinh Nguyen <Dinh.Nguyen@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>