nxp: ls1046ardb: Convert README to rST

This converts the readme for this board to rST. I have tried not to
change any semantics from the original (though I did convert MB to M).

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
diff --git a/board/freescale/ls1046ardb/MAINTAINERS b/board/freescale/ls1046ardb/MAINTAINERS
index efdea22..3c8cfe7 100644
--- a/board/freescale/ls1046ardb/MAINTAINERS
+++ b/board/freescale/ls1046ardb/MAINTAINERS
@@ -14,3 +14,4 @@
 F:	configs/ls1046ardb_SECURE_BOOT_defconfig
 F:	configs/ls1046ardb_sdcard_SECURE_BOOT_defconfig
 F:	configs/ls1046ardb_qspi_SECURE_BOOT_defconfig
+F:	doc/board/nxp/ls1046ardb.rst