riscv: save hart ID in register tp instead of s0

riscv: save hart ID in register tp instead of s0

The hart ID passed by the previous boot stage is currently stored in
register s0. If we divert the control flow inside a function, which is
required as part of multi-hart support, the function epilog may not be
called, clobbering register s0. Save the hart ID in the unallocatable
register tp instead to protect the hart ID.

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