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/k2g_evm_defconfig b/configs/k2g_evm_defconfig
index 9b54ebb..54ce875 100644
--- a/configs/k2g_evm_defconfig
+++ b/configs/k2g_evm_defconfig
@@ -12,6 +12,7 @@
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_TARGET_K2G_EVM=y
 CONFIG_ENV_SIZE=0x40000
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_SPL_DM_SPI=y
 CONFIG_DEFAULT_DEVICE_TREE="keystone-k2g-evm"
 CONFIG_SPL_SERIAL_SUPPORT=y