riscv: fix inconsistent use of spaces and tabs in start.S

start.S uses both tabs and spaces after instructions. Fix this by only
using tabs after instructions.

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>