target/hppa: Add privilege to MMU index conversion helpers

target/hppa: Add privilege to MMU index conversion helpers

Add two macros which convert privilege level to/from MMU index:

- PRIV_TO_MMU_IDX(priv)
    returns the MMU index for the given privilege level

- MMU_IDX_TO_PRIV(mmu_idx)
    returns the corresponding privilege level for this MMU index

The introduction of those macros make the code easier to read and
will help to improve performance in follow-up patch.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>