boards: nxp: rt1170_evk: Add XMCD bootheader

Currently, only DCD bootheader was supported to configure the SDRAM.

On IMX RT1170, XMCD can be used as an alternative boot header to DCD.
XMCD is more advanced than DCD and enhances SDRAM access speed.
This is benefit for SDRAM access application.

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