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/devkit3250_defconfig b/configs/devkit3250_defconfig
index ff21844..1d089bd 100644
--- a/configs/devkit3250_defconfig
+++ b/configs/devkit3250_defconfig
@@ -10,6 +10,7 @@
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_ENV_SIZE=0x20000
 CONFIG_ENV_OFFSET=0xA0000
+CONFIG_SYS_MALLOC_LEN=0x100000
 CONFIG_SPL_TEXT_BASE=0x00000000
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL=y