boards: imx8mm/n/p imx93: add U-Boot requirement in document

Currently NXP MPU depends on "cpu" command in uboot to kick Zephyr to
Cortex-A Core, added U-Boot version which support "cpu" command in the
board documents.

Refined boot commands.

Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>