riscv: Remove CSR read/write defines in encoding.h

riscv: Remove CSR read/write defines in encoding.h

There is no reason to keep two versions of CSR read/write defines
in encoding.h. We already have one set of defines in csr.h, which
is from Linux kernel, and let's drop the one in encoding.h.

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