riscv: qemu: define standalone load address

We need to define the standalone load address to use standalone
application on qemu-riscv. Define it and set it equal to
CONFIG_SYS_LOAD_ADDR.

To not overwrite it, change the assigned of CONFIG_STANDALONE_LOAD_ADDR
in arch/riscv/config.mk to a conditional one.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>