doc: board: ti: j784s4: document OSPI NOR layout

Added OSPI NOR flash layout diagram, as well as example commands to flash
firmware to it.  Added OSPI boot mode pin setting.

Signed-off-by: Jonathan Humphreys <j-humphreys@ti.com>
diff --git a/doc/board/ti/j784s4_evm.rst b/doc/board/ti/j784s4_evm.rst
index 5c4bd2c..2ffec3d 100644
--- a/doc/board/ti/j784s4_evm.rst
+++ b/doc/board/ti/j784s4_evm.rst
@@ -153,6 +153,31 @@
 .. image:: img/dm_tispl.bin.svg
     :alt: tispl.bin format
 
+OSPI:
+-----
+ROM supports booting from OSPI from offset 0x0.
+
+Flashing images to OSPI NOR:
+
+Below commands can be used to download tiboot3.bin, tispl.bin, and
+u-boot.img over tftp and then flash those to OSPI at their respective
+addresses.
+
+.. prompt:: bash =>
+
+  sf probe
+  tftp ${loadaddr} tiboot3.bin
+  sf update $loadaddr 0x0 $filesize
+  tftp ${loadaddr} tispl.bin
+  sf update $loadaddr 0x80000 $filesize
+  tftp ${loadaddr} u-boot.img
+  sf update $loadaddr 0x280000 $filesize
+
+Flash layout for OSPI NOR:
+
+.. image:: img/ospi_sysfw3.svg
+  :alt: OSPI NOR flash partition layout
+
 R5 Memory Map
 -------------
 
@@ -262,6 +287,9 @@
    * - SD
      - 0000
 
+   * - OSPI
+     - 0010
+
    * - EMMC
      - 0110