RISC-V: pi: Add kernel/pi/pi.h

Add pi.h header for declarations of the kernel/pi prefixed functions
and any other related declarations.

Suggested-by: Charlie Jenkins <charlie@rivosinc.com>
Signed-off-by: Jesse Taube <jesse@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240709173937.510084-4-jesse@rivosinc.com


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