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/devkit8000_defconfig b/configs/devkit8000_defconfig
index 4e15bed..0ec943a 100644
--- a/configs/devkit8000_defconfig
+++ b/configs/devkit8000_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_OMAP2PLUS=y
 CONFIG_SYS_TEXT_BASE=0x80100000
 CONFIG_NR_DRAM_BANKS=2
+CONFIG_SYS_MALLOC_LEN=0x40000
 CONFIG_SPL_TEXT_BASE=0x40200000
 CONFIG_TARGET_DEVKIT8000=y
 CONFIG_SPL=y