riscv: save hart ID and device tree passed by prior boot stage

Store the hart ID and device tree passed by the prior boot stage (in a0
and a1) in registers s0 and s1. Replace one use of s1 in start.S to
avoid overwriting it.

The device tree is also stored in memory to make it available to U-Boot
with the configuration CONFIG_OF_PRIOR_STAGE.

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