lib: sbi: protect dprintf output with spinlock

Avoid getting messages from multiple harts (using dprintf and printf)
concurrently with a spinlock serializaing calls to sbi_dprintf(),
sbi_printf() and sbi_puts()

Signed-off-by: Dong Du <Dd_nirvana@sjtu.edu.cn>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>