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/imx8qm_mek_defconfig b/configs/imx8qm_mek_defconfig
index 3a12354..516a83b 100644
--- a/configs/imx8qm_mek_defconfig
+++ b/configs/imx8qm_mek_defconfig
@@ -8,6 +8,7 @@
 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="fsl-imx8qm-mek"
 CONFIG_SPL_TEXT_BASE=0x100000