lib: sbi: Add BUG() macro for csr_read/write_num() and misa_string()

We use BUG() macro in csr_read_num(), csr_write_num(), and
misa_string() functions for unhandled cases.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>