[RISCV][NFC] Add missing immediate operand types.

Some immediate types in RISCV target description lack operand type field.
This leads them being listed as OPERAND_UNKNOWN in MCOperandInfo. This patch adds this fields.
This is NFC because it does not affect flow of any current tools implementation.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D144105