Fix sprz319 erratum 2.1

There is an erratum in DM3730 which results in the
EHCI USB PLL (DPLL5) not updating sufficiently frequently; this
leads to USB PHY clock drift and once the clock has drifted far
enough, the PHY's ULPI interface stops responding and USB
drops out. This is manifested on a Beagle xM by having the attached
SMSC9514 report 'Cannot enable port 2. Maybe the USB cable is bad?'
or similar.

The fix is to carefully adjust your DPLL5 settings so as to
keep the PHY clock as close as possible to 120MHz over the long
term; TI SPRZ319e gives a table of such settings and this patch
applies that table to systems with a 13MHz or a 26MHz clock,
thus fixing the issue (inasfar as it can be fixed) on Beagle xM
and Overo Firestorm.

Signed-off-by: Richard Watts <rrw@kynesim.co.uk>