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/xilinx_zynqmp_mini_nand_single_defconfig b/configs/xilinx_zynqmp_mini_nand_single_defconfig
index 9d7cec2..34fd347 100644
--- a/configs/xilinx_zynqmp_mini_nand_single_defconfig
+++ b/configs/xilinx_zynqmp_mini_nand_single_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SYS_TEXT_BASE=0x10000
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x80
+CONFIG_SYS_MALLOC_LEN=0x800000
 CONFIG_DEFAULT_DEVICE_TREE="zynqmp-mini-nand"
 # CONFIG_CMD_ZYNQMP is not set
 CONFIG_SYS_LOAD_ADDR=0x8000000