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/poplar_defconfig b/configs/poplar_defconfig
index 8c7a565..007d116 100644
--- a/configs/poplar_defconfig
+++ b/configs/poplar_defconfig
@@ -5,6 +5,7 @@
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_ENV_SIZE=0x10000
 CONFIG_ENV_OFFSET=0x1F0000
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_DEFAULT_DEVICE_TREE="hi3798cv200-poplar"
 CONFIG_IDENT_STRING="poplar"
 CONFIG_DISTRO_DEFAULTS=y