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/xtfpga_defconfig b/configs/xtfpga_defconfig
index 431267f..cd5425a 100644
--- a/configs/xtfpga_defconfig
+++ b/configs/xtfpga_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_CPU="dc233c"
 CONFIG_ENV_SIZE=0x20000
 CONFIG_ENV_SECT_SIZE=0x20000
+CONFIG_SYS_MALLOC_LEN=0x40000
 CONFIG_XTFPGA_KC705=y
 CONFIG_SYS_LOAD_ADDR=0x02000000
 CONFIG_SHOW_BOOT_PROGRESS=y