ARM: mxs: fix VDDD brownout setting

At the moment, the desired brownout is at 1.0V. However,
this setting cannot be realized by hardware since we have
only 3 bits to represent the voltage difference from the
target value.

Target value is 1500 mV, brownout target is 1000 mV,
voltage steps are 25 mV.

Register content calculation:
  (1500 [mV] - 1000 [mV]) / 25 [mV] = 20 (decimal) = 0x14

  Register takes only 3 bits, that is 0x4.

But 0x4 * 25 [mV] = 100 [mV], that means that actual
brownout level is 1500 [mV] - 100 [mV] = 1.4 V.

Minimum possible BO level is
  1500 [mV] - 0x7 * 25 [mV] = 1315 [mV].

So lets use this value as desired BO value (which is
also the same as FSL bootlets use).

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>

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