Convert CONFIG_CMD_SPL_WRITE_SIZE to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_SPL_WRITE_SIZE

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/configs/ipam390_defconfig b/configs/ipam390_defconfig
index 702b311..80d9f47 100644
--- a/configs/ipam390_defconfig
+++ b/configs/ipam390_defconfig
@@ -19,6 +19,7 @@
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0x00180000
+CONFIG_CMD_SPL_WRITE_SIZE=0x400
 CONFIG_CMD_ASKENV=y
 CONFIG_CRC32_VERIFY=y
 # CONFIG_CMD_FLASH is not set