include: Make sbi_hart_id_to_scratch() as macro

The sbi_hart_id_to_scratch() just forwards call to firmware specific
hartid_to_scratch() callback so we make sbi_hart_id_to_scratch() as
macro in sbi_scratch.h instead of regular function in sbi_hart.c.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>