riscv: Access CSRs using CSR numbers

riscv: Access CSRs using CSR numbers

We should prefer accessing CSRs using their CSR numbers
because:
1. It compiles fine with older toolchains.
2. We can use latest CSR names in #define macro names of CSR
   numbers as-per RISC-V spec.
3. We can access newly added CSRs even if toolchain does not
   recognize newly added CSRs by name.

This commit is inspired from Linux kernel commit a3182c91ef4e
("RISC-V: Access CSRs using CSR numbers").

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>