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/apalis-imx8_defconfig b/configs/apalis-imx8_defconfig
index e0cad08..87c9842 100644
--- a/configs/apalis-imx8_defconfig
+++ b/configs/apalis-imx8_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SYS_MEMTEST_END=0x89000000
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0xFFFFDE00
+CONFIG_SYS_MALLOC_LEN=0x2800000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="fsl-imx8qm-apalis"
 CONFIG_TARGET_APALIS_IMX8=y