DaVinci: fix ddr2 vtp i/o calibration

DaVinci: fix ddr2 vtp i/o calibration

Previously, only the low 5 bits (NCH) were being transfered
from DDRVTPR to DDRVTPIOCR, the bits 5-9 where zeroed.

VTP_RECAL should be bit 15, not 18.

The only mainline board affected by this change is davinci_sonata.
The other Davinci boards define CONFIG_SKIP_LOWLEVEL_INIT.

However, if the program that loads u-boot on these boards
copied the code from u-boot, they will need fixed as well.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Please get tested by acks before applying, where tested by
means an overnight memory test.

Thanks
Troy