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_arria10_defconfig b/configs/socfpga_arria10_defconfig
index ef9bbb9..bf793d9 100644
--- a/configs/socfpga_arria10_defconfig
+++ b/configs/socfpga_arria10_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_SOCFPGA=y
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0x4400
+CONFIG_SYS_MALLOC_LEN=0x4000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="socfpga_arria10_socdk_sdmmc"
 CONFIG_SPL_TEXT_BASE=0xFFE00000