riscv: mm: Add pfn_to_kaddr() implementation

The pfn_to_kaddr() function is used by KASAN's memory hotplugging
path. Add the missing function to the RISC-V port, so that it can be
built with MHP and CONFIG_KASAN.

Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240605114100.315918-6-bjorn@kernel.org


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