riscv: Sync csr.h with Linux kernel v5.2

This syncs csr.h with Linux kernel 5.2, and imports asm.h that
is required by csr.h.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>