riscv: qemu: add SPL configuration

riscv: qemu: add SPL configuration

Add two new configurations (qemu-riscv{32,64}_spl_defconfig) with SPL
enabled for RISC-V QEMU. QEMU does not require SPL to run U-Boot. The
configurations are meant to help the development of SPL on RISC-V.

The configurations enable RAM as the only SPL boot device. Images must
be loaded at address 0x80200000. In the default boot flow, U-Boot SPL
starts in machine mode, loads the OpenSBI FW_DYNAMIC firmware and U-Boot
proper from the supplied FIT image, and starts OpenSBI. U-Boot proper is
then started in supervisor mode by OpenSBI.

Signed-off-by: Lukas Auer <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Tested-by: Bin Meng <[email protected]>
Reviewed-by: Anup Patel <[email protected]>