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_defconfig b/configs/mx6slevk_defconfig
index 5eb50a0..c041cb7 100644
--- a/configs/mx6slevk_defconfig
+++ b/configs/mx6slevk_defconfig
@@ -6,6 +6,7 @@
 CONFIG_ENV_OFFSET=0x80000
 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