board: developerbox: update flash rawwrite binary size

Current documentation limits the firmware size to 1.5MB.
When the fTPM and StandaloneMM-based RPMB secure storage is
enabled, firmware size is bigger than that size.
Let's specify the A/B update bank size(4MB) for flash
rawwrite parameter.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/doc/board/socionext/developerbox.rst b/doc/board/socionext/developerbox.rst
index 596cf9f..46712c3 100644
--- a/doc/board/socionext/developerbox.rst
+++ b/doc/board/socionext/developerbox.rst
@@ -212,8 +212,8 @@
 Once the flasher tool is running we are ready to flash the images.::
 Write the FIP image to the Bank-0 & 1 at 6MB and 10MB offset.::
 
-  flash rawwrite 600000 180000
-  flash rawwrite a00000 180000
+  flash rawwrite 600000 400000
+  flash rawwrite a00000 400000
   >> Send SPI_NOR_NEWFIP.fd via XMODEM (Control-A S in minicom) <<
 
   flash rawwrite 500000 1000