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/mx6sabresd_defconfig b/configs/mx6sabresd_defconfig
index 02e1d6a..a5e63aa 100644
--- a/configs/mx6sabresd_defconfig
+++ b/configs/mx6sabresd_defconfig
@@ -23,6 +23,7 @@
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPL=y
+CONFIG_CMD_SPL_WRITE_SIZE=0x20000
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_MMC=y
 CONFIG_CMD_PART=y