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/cgtqmx8_defconfig b/configs/cgtqmx8_defconfig
index 2020364..c1ec196 100644
--- a/configs/cgtqmx8_defconfig
+++ b/configs/cgtqmx8_defconfig
@@ -9,6 +9,7 @@
 CONFIG_NR_DRAM_BANKS=3
 CONFIG_ENV_SIZE=0x1000
 CONFIG_ENV_OFFSET=0x400000
+CONFIG_SYS_MALLOC_LEN=0x2400000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx8qm-cgtqmx8"
 CONFIG_TARGET_CONGA_QMX8=y