arm: mxs: Increase VDDD voltage to match specification

According to IMX28CEC rev. 4, 10/2018, Table 15. Recommended Operating
Conditions, page 16, the VDDD should be set to 1.55V when the CPU is
operating at 454MHz. This is the case in U-Boot, hence increase the
VDDD voltage. This fixes instability when performing TFTP transfers.
Increase the brownout threshold to 1.4V. The documentation recommends
1.45V setting for the brownout, however, this triggers failure during
power block init, so keep the brownout slightly lower.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>