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/stemmy_defconfig b/configs/stemmy_defconfig
index 975a12a..c026081 100644
--- a/configs/stemmy_defconfig
+++ b/configs/stemmy_defconfig
@@ -3,6 +3,7 @@
 CONFIG_ARCH_U8500=y
 CONFIG_SYS_TEXT_BASE=0x100000
 CONFIG_NR_DRAM_BANKS=2
+CONFIG_SYS_MALLOC_LEN=0x0200000
 CONFIG_DEFAULT_DEVICE_TREE="ste-ux500-samsung-stemmy"
 CONFIG_SYS_LOAD_ADDR=0x100000
 CONFIG_SYS_CONSOLE_INFO_QUIET=y