at91: fix a USB problem for AT91SAM9261

This patch corrects the missing PLLB initialization in usb_cpu_init()
for AT91SAM9261.
Because of the missing PLLB initialization, the USB support for all
AT91SAM9261 based boards will work only if the PLLB is configured by a
precedent bootloader.

Signed-off-by: Ilko Iliev <iliev@ronetix.at>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Remy Bohmer <linux@bohmer.net>