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/edminiv2_defconfig b/configs/edminiv2_defconfig
index d3bde66..c5890e2 100644
--- a/configs/edminiv2_defconfig
+++ b/configs/edminiv2_defconfig
@@ -8,6 +8,7 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_SECT_SIZE=0x2000
+CONFIG_SYS_MALLOC_LEN=0x40000
 CONFIG_SPL_TEXT_BASE=0xffff0000
 CONFIG_TARGET_EDMINIV2=y
 CONFIG_SPL_SERIAL_SUPPORT=y