at91sam3: Wrong PLLA frequency calculations

The command 'at91sam3 info' ignores PLLA DIV values >1. This patch fixes it.
Tested on a SAM3S4C chip.

Change-Id: I051f41bb3dcefe1ac785fbcb48477a807daa16a2
Signed-off-by: Thomas Schmid <thomas.schmid@gmail.com>
Reviewed-on: http://openocd.zylin.com/1307
Tested-by: jenkins
Reviewed-by: Peter Stuge <peter@stuge.se>