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_stratix10_atf_defconfig b/configs/socfpga_stratix10_atf_defconfig
index 4a36647..ff0373e 100644
--- a/configs/socfpga_stratix10_atf_defconfig
+++ b/configs/socfpga_stratix10_atf_defconfig
@@ -6,6 +6,7 @@
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_ENV_SIZE=0x1000
 CONFIG_ENV_OFFSET=0x200
+CONFIG_SYS_MALLOC_LEN=0x500000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="socfpga_stratix10_socdk"
 CONFIG_SPL_TEXT_BASE=0xFFE00000