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/qemu-riscv64_spl_defconfig b/configs/qemu-riscv64_spl_defconfig
index 7826102..5cf72d7 100644
--- a/configs/qemu-riscv64_spl_defconfig
+++ b/configs/qemu-riscv64_spl_defconfig
@@ -1,6 +1,7 @@
 CONFIG_RISCV=y
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x20000
+CONFIG_SYS_MALLOC_LEN=0x800000
 CONFIG_DEFAULT_DEVICE_TREE="qemu-virt"
 CONFIG_SPL=y
 CONFIG_TARGET_QEMU_VIRT=y