board: ti: am43xx: take care of all OPPs

Make sure that all OPPs are checked on
scale_vcores(). While at that also fix 600MHz
VDD_MPU voltage according to AM437x Data Manual
available at [1].

Table 5-3 on that document, lists all valid
voltages per frequency.

[1] http://www.ti.com/lit/ds/symlink/am4379.pdf



Signed-off-by: Felipe Balbi <balbi@ti.com>