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/imx6q_logic_defconfig b/configs/imx6q_logic_defconfig
index a752382..971140f 100644
--- a/configs/imx6q_logic_defconfig
+++ b/configs/imx6q_logic_defconfig
@@ -12,6 +12,7 @@
 CONFIG_MX6Q=y
 CONFIG_MX6_OCRAM_256KB=y
 CONFIG_TARGET_MX6LOGICPD=y
+CONFIG_SYS_MALLOC_LEN=0xa00000
 CONFIG_DEFAULT_DEVICE_TREE="imx6q-logicpd"
 CONFIG_SPL_TEXT_BASE=0x00908000
 CONFIG_SPL_MMC_SUPPORT=y