doc: board: raspberrypi: Add documentation

Add documentation about the different configuration files for the
RaspberryPi board family.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/board/broadcom/index.rst

+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright (C) 2022 Matthias Brugger <mbrugger@suse.com>
+
+Broadcom
+========
+
+.. toctree::
+   :maxdepth: 2
+
+   raspberrypi

doc/board/broadcom/raspberrypi.rst

+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright (C) 2022 Matthias Brugger <mbrugger@suse.com>
+
+Raspberry Pi
+============
+
+About this
+----------
+
+This document describes the information about Raspberry Pi boards
+and it's usage steps.
+
+Raspberry Pi boards
+-------------------
+
+List of the supported Rasbperry Pi boards and the corresponding defconfig files:
+
+32 bit
+^^^^^^
+
+* rpi_defconfig
+  - Raspberry Pi
+* rpi_0_w_defconfig
+  - Raspberry Pi 1
+  - Raspberry Pi zero
+* rpi_2_defconfig
+  - Raspberry Pi 2
+* rpi_3_32b_defconfig
+  - Raspberry Pi 3b
+* rpi_4_32b_defconfig
+  - Raspberry Pi 4b
+
+64 bit
+^^^^^^
+
+* rpi_3_defconfig
+  - Raspberry Pi 3b
+* rpi_3_b_plus_defconfig
+  - Raspberry Pi 3b+
+* rpi_4_defconfig
+  - Raspberry Pi 4b
+* rpi_arm64_defconfig
+  - Raspberry Pi 3b
+  - Raspberry Pi 3b+
+  - Raspberry Pi 4b
+  - Raspberry Pi 400
+  - Raspberry Pi CM 3
+  - Raspberry Pi CM 3+
+  - Raspberry Pi CM 4
+  - Raspberry Pi zero 2 w
+
+rpi_arm64_defconfig uses the device-tree provided by the firmware instead of
+the embedded one. It allows to use the same U-Boot binary to boot different
+boards.

doc/board/index.rst

@@ -14,6 +14,7 @@ Board-specific doc
    apple/index
    armltd/index
    atmel/index
+   broadcom/index
    congatec/index
    coreboot/index
    emulation/index