mx51: fix usb clock support

Current code doesn't really enable the usb clocks so if they're disabled
when booting linux, the kernel/machine will hang as soon as someone is trying
to read a usb register

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