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/ae350_rv32_defconfig b/configs/ae350_rv32_defconfig
index a3b2a4d..dab35f9 100644
--- a/configs/ae350_rv32_defconfig
+++ b/configs/ae350_rv32_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_TEXT_BASE=0x00000000
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_ENV_SECT_SIZE=0x1000
+CONFIG_SYS_MALLOC_LEN=0x80000
 CONFIG_DEFAULT_DEVICE_TREE="ae350_32"
 CONFIG_TARGET_AX25_AE350=y
 CONFIG_DISTRO_DEFAULTS=y