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/gwventana_gw5904_defconfig b/configs/gwventana_gw5904_defconfig
index 7830d90..5b2672c 100644
--- a/configs/gwventana_gw5904_defconfig
+++ b/configs/gwventana_gw5904_defconfig
@@ -33,6 +33,7 @@
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPL_NAND_OFS=0x1100000
+CONFIG_CMD_SPL_WRITE_SIZE=0x20000
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y
 CONFIG_CMD_I2C=y