riscv: do not rely on hart ID passed by previous boot stage

riscv: do not rely on hart ID passed by previous boot stage

RISC-V U-Boot expects the hart ID to be passed to it via register a0 by
the previous boot stage. Machine mode firmware such as BBL and OpenSBI
do this when starting their payload (U-Boot) in supervisor mode. If
U-Boot is running in machine mode, this task must be handled by the boot
ROM. Explicitly populate register a0 with the hart ID from the mhartid
CSR to avoid possible problems on RISC-V processors with a boot ROM that
does not handle this task.

Suggested-by: Rick Chen <[email protected]>
Signed-off-by: Lukas Auer <[email protected]>
Reviewed-by: Anup Patel <[email protected]>
Reviewed-by: Atish Patra <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Tested-by: Bin Meng <[email protected]>
Reviewed-by: Rick Chen <[email protected]>
Tested-by: Rick Chen <[email protected]>