sunxi: correct documentation for SPI flashing

The mtd_debug write does not work in this context. The flashcp command does
work, provides both the erase and write functions and with the verbose
option gives good feedback.

Signed-off-by: Stephen Graf <stephen.graf@gmail.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/doc/board/allwinner/sunxi.rst b/doc/board/allwinner/sunxi.rst
index 797222d..d0c89b9 100644
--- a/doc/board/allwinner/sunxi.rst
+++ b/doc/board/allwinner/sunxi.rst
@@ -251,8 +251,7 @@
 
     # apt-get install mtd-utils
     # mtdinfo
-    # mtd_debug erase /dev/mtdX 0 0xf0000
-    # mtd_debug write /dev/mtdX 0 0xf0000 u-boot-sunxi-with-spl.bin
+    # flashcp -v u-boot-sunxi-with-spl.bin /dev/mtdX
 
 ``/dev/mtdX`` needs to be replaced with the respective device name, as listed
 in the output of ``mtdinfo``.