riscv: bootm: Change to use boot_hart from global data

riscv: bootm: Change to use boot_hart from global data

Avoid reading mhartid CSR directly, instead use the one we saved
in the global data structure before.

With this patch, BBL no longer needs to be hacked to provide the
mhartid CSR emulation for S-mode U-Boot.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Anup Patel <anup@brainfault.org>