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/P5040DS_SDCARD_defconfig b/configs/P5040DS_SDCARD_defconfig
index c8a495a..a1b4cf3 100644
--- a/configs/P5040DS_SDCARD_defconfig
+++ b/configs/P5040DS_SDCARD_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_TEXT_BASE=0xFFF40000
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0xCF400
+CONFIG_SYS_MALLOC_LEN=0x100000
 CONFIG_DEFAULT_DEVICE_TREE="p5040ds"
 CONFIG_MPC85xx=y
 CONFIG_TARGET_P5040DS=y