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/ls1028ardb_tfa_SECURE_BOOT_defconfig b/configs/ls1028ardb_tfa_SECURE_BOOT_defconfig
index ad61d60..995cfa6 100644
--- a/configs/ls1028ardb_tfa_SECURE_BOOT_defconfig
+++ b/configs/ls1028ardb_tfa_SECURE_BOOT_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SYS_MEMTEST_END=0x9fffffff
 CONFIG_ENV_SIZE=0x2000
 CONFIG_NXP_ESBC=y
+CONFIG_SYS_MALLOC_LEN=0x202000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="fsl-ls1028a-rdb"
 CONFIG_FSPI_AHB_EN_4BYTE=y