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/am43xx_evm_defconfig b/configs/am43xx_evm_defconfig
index a66d807..afac014 100644
--- a/configs/am43xx_evm_defconfig
+++ b/configs/am43xx_evm_defconfig
@@ -17,6 +17,7 @@
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0x00100000
+CONFIG_CMD_SPL_WRITE_SIZE=0x40000
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_NAND=y
 # CONFIG_CMD_SETEXPR is not set