powerpc/t4240qds: Add VDD override

Allow VDD voltage overriding with a command. This is an add-on feasture of
VID. To override VDD, use command vdd_override with the value of voltage
in mV, for example

vdd_override <voltage in mV, eg. 1050>

The above example will set the VDD to 1.050 volt. Any wrong value out of
range of 0.8188 to 1.2125 volt or invalid string is ignored.

In addition to the command, if overriding VDD is needed earlier in booting
process, save an variable and reboot:

setenv t4240qds_vdd_mv <voltage in mV>
saveenv

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>