Convert CONFIG_SYS_MALLOC_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MALLOC_LEN

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/configs/udoo_neo_defconfig b/configs/udoo_neo_defconfig
index b689511..a713ad5 100644
--- a/configs/udoo_neo_defconfig
+++ b/configs/udoo_neo_defconfig
@@ -10,6 +10,7 @@
 CONFIG_MX6SX=y
 CONFIG_TARGET_UDOO_NEO=y
 CONFIG_SYS_I2C_MXC_I2C1=y
+CONFIG_SYS_MALLOC_LEN=0x300000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx6sx-udoo-neo-basic"
 CONFIG_SPL_TEXT_BASE=0x00908000