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_defconfig b/configs/xilinx_zynqmp_mini_defconfig
index e3d94a8..736586e 100644
--- a/configs/xilinx_zynqmp_mini_defconfig
+++ b/configs/xilinx_zynqmp_mini_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SYS_MEMTEST_START=0x00000000
 CONFIG_SYS_MEMTEST_END=0x00001000
 CONFIG_ENV_SIZE=0x80
+CONFIG_SYS_MALLOC_LEN=0x1a00
 CONFIG_DEFAULT_DEVICE_TREE="zynqmp-mini"
 CONFIG_SYS_MEM_RSVD_FOR_MMU=y
 CONFIG_ZYNQMP_PSU_INIT_ENABLED=y