doc: board: nxp: Add remoteproc guide

Add guide on how to use the Remote Processors on i.MX8M and i.MX93.

Update MAINTAINERS to include doc/board/nxp.

Signed-off-by: Peng Fan <peng.fan@nxp.com>

MAINTAINERS

@@ -317,6 +317,7 @@ F:	arch/arm/include/asm/mach-imx/
 F:	board/freescale/*mx*/
 F:	board/freescale/common/
 F:	common/spl/spl_imx_container.c
+F:	doc/board/nxp/
 F:	doc/imx/
 F:	drivers/mailbox/imx-mailbox.c
 F:	drivers/remoteproc/imx*

doc/board/nxp/index.rst

@@ -26,4 +26,5 @@ NXP Semiconductors
    mx6sabresd
    mx6ul_14x14_evk
    mx6ullevk
+   rproc
    psb

doc/board/nxp/rproc.rst

@@ -0,0 +1,69 @@
+.. SPDX-License-Identifier: GPL-2.0+
+   Copyright 2025 NXP
+   Written by Peng Fan <peng.fan@nxp.com>
+
+i.MX remoteproc usage guide
+===========================
+
+Introduction
+------------
+
+This guide is for giving user how to use the Remote Processors found on
+various i.MX Chips. The term remote processor is indicating the Cortex-M
+[4,7,33] cores inside i.MX family.
+
+i.MX8MM-EVK
+-----------
+
+Steps to start the Cortex-M4 core
+
+    load mmc 2:2 0x90000000 /lib/firmware/imx8mm_m4_TCM_rpmsg_lite_str_echo_rtos.elf
+
+    rproc load 0 0x90000000 ${filesize}
+
+    rproc start 0
+
+i.MX8MN-EVK
+-----------
+
+Steps to start the Cortex-M7 core
+
+    load mmc 2:2 0x90000000 /lib/firmware/imx8mn_m7_TCM_rpmsg_lite_str_echo_rtos.elf
+
+    rproc load 0 0x90000000 ${filesize}
+
+    rproc start 0
+
+i.MX8MQ-EVK
+-----------
+
+Steps to start the Cortex-M4 core
+
+    load mmc 0:2 0x90000000 /lib/firmware/imx8mq_m4_TCM_rpmsg_lite_str_echo_rtos.elf
+
+    rproc load 0 0x90000000 ${filesize}
+
+    rproc start 0
+
+i.MX8MP-EVK
+-----------
+
+Steps to start the Cortex-M7 core
+
+    load mmc 2:2 0x90000000 /lib/firmware/imx8mp_m7_TCM_rpmsg_lite_str_echo_rtos.elf
+
+    rproc load 0 0x90000000 ${filesize}
+
+    rproc start 0
+
+i.MX93-FRDM/QSB/EVK
+-------------------
+
+Steps to start the Cortex-M33 core, need to choose the correct file for
+corresponding board.
+
+    load mmc 0:2 0x90000000 /lib/firmware/imx93-11x11-evk_m33_TCM_rpmsg_lite_str_echo_rtos.elf
+
+    rproc load 0 0x90000000 ${filesize}
+
+    rproc start 0