riscv: cpu: set gp before board_init_f_init_reserve

Restore global pointer before board_init_f_init_reserve call,
as "a0" can be set in harts_early_init call and we end up with
invalid global pointer.

Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>