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/pico-imx6_defconfig b/configs/pico-imx6_defconfig
index 588ddd0..c080bdd 100644
--- a/configs/pico-imx6_defconfig
+++ b/configs/pico-imx6_defconfig
@@ -9,6 +9,7 @@
 CONFIG_ENV_OFFSET=0xC0000
 CONFIG_MX6QDL=y
 CONFIG_TARGET_PICO_IMX6=y
+CONFIG_SYS_MALLOC_LEN=0x2300000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx6dl-pico"
 CONFIG_SPL_TEXT_BASE=0x00908000