board: gateworks: venice: update board doc to show other emmc parts

Update the venice board documentation to show how to install to the
various eMMC hardware partitions available as the same binary firmware
can be placed in either user/boot0/boot1 without build-time config
changes. Note that the boot offsets differ depending on the SoC and the
eMMC hardware partition.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
diff --git a/doc/board/gateworks/imx8mp_venice.rst b/doc/board/gateworks/imx8mp_venice.rst
index 632cd74..a219caa 100644
--- a/doc/board/gateworks/imx8mp_venice.rst
+++ b/doc/board/gateworks/imx8mp_venice.rst
@@ -47,4 +47,6 @@
 
    => tftpboot $loadaddr flash.bin
    => setexpr blkcnt $filesize + 0x1ff && setexpr blkcnt $blkcnt / 0x200
-   => mmc dev 2 && mmc write $loadaddr 0x40 $blkcnt
+   => mmc dev 2 0 && mmc write $loadaddr 0x40 $blkcnt # emmc user hw part
+   => mmc dev 2 1 && mmc write $loadaddr 0 $blkcnt # or emmc boot0 hw part
+   => mmc dev 2 2 && mmc write $loadaddr 0 $blkcnt # or emmc boot1 hw part