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/stih410-b2260_defconfig b/configs/stih410-b2260_defconfig
index aa63d0d..d5ec00e 100644
--- a/configs/stih410-b2260_defconfig
+++ b/configs/stih410-b2260_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_TEXT_BASE=0x7D600000
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x4000
+CONFIG_SYS_MALLOC_LEN=0x1800000
 CONFIG_DEFAULT_DEVICE_TREE="stih410-b2260"
 CONFIG_IDENT_STRING="STMicroelectronics STiH410-B2260"
 CONFIG_DISTRO_DEFAULTS=y