Documentation/kernel-parameters: Add riscv unaligned speed parameters

Document riscv parameters used to select scalar and vector unaligned
access speeds.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20250304120014.143628-18-ajones@ventanamicro.com


Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>