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/imgtec_xilfpga_defconfig b/configs/imgtec_xilfpga_defconfig
index ced7e03..79c5758 100644
--- a/configs/imgtec_xilfpga_defconfig
+++ b/configs/imgtec_xilfpga_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SKIP_LOWLEVEL_INIT=y
 CONFIG_SYS_MALLOC_F_LEN=0x600
 CONFIG_ENV_SIZE=0x4000
+CONFIG_SYS_MALLOC_LEN=0x40000
 CONFIG_DEFAULT_DEVICE_TREE="nexys4ddr"
 CONFIG_TARGET_XILFPGA=y
 CONFIG_MIPS_CACHE_SETUP=y