doc: imx8mp_evk: Use in-tree build in the example

To make it consistent with the instructions from other NXP imx8m boards,
such as imx8mm-evk and imx8mn-evk, use U-Boot in-tree build in the
examples.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
1 file changed