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/mx6slevk_spinor_defconfig b/configs/mx6slevk_spinor_defconfig
index e8801de..1e6aae6 100644
--- a/configs/mx6slevk_spinor_defconfig
+++ b/configs/mx6slevk_spinor_defconfig
@@ -7,6 +7,7 @@
 CONFIG_ENV_SECT_SIZE=0x10000
 CONFIG_MX6SL=y
 CONFIG_TARGET_MX6SLEVK=y
+CONFIG_SYS_MALLOC_LEN=0x300000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx6sl-evk"
 # CONFIG_CMD_BMODE is not set