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>