boards: nxp: rt1170_evk: Enable XMCD as bootheader by default

Configure to use XMCD as the bootheader by default instead of DCD. As
XMCD gives a better SDRAM memory access speed.
As an example, the Pixel Processing Pipeline is 13 ms faster with XMCD
compared to DCD as the bootheader.

Signed-off-by: Trung Hieu Le <trunghieu.le@nxp.com>