RISC-V: add missing single letter extension definitions

To facilitate adding single letter extensions to riscv_isa_ext, add
definitions for the extensions present in base_riscv_exts that do not
already have them.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Evan Green <evan@rivosinc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230713-train-feisty-93de38250f98@wendy


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>