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/socfpga_secu1_defconfig b/configs/socfpga_secu1_defconfig
index 2f2db97..63cbf31 100644
--- a/configs/socfpga_secu1_defconfig
+++ b/configs/socfpga_secu1_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x800
 CONFIG_ENV_SIZE=0x1000
 CONFIG_ENV_OFFSET=0x100000
+CONFIG_SYS_MALLOC_LEN=0x4000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="socfpga_arria5_secu1"
 CONFIG_SPL_TEXT_BASE=0xFFFF0000