target/ppc: Streamline calculation of RMA limit from LPCR[RMLS]

target/ppc: Streamline calculation of RMA limit from LPCR[RMLS]

Currently we use a big switch statement in ppc_hash64_update_rmls() to work
out what the right RMA limit is based on the LPCR[RMLS] field.  There's no
formula for this - it's just an arbitrary mapping defined by the existing
CPU implementations - but we can make it a bit more readable by using a
lookup table rather than a switch.  In addition we can use the MiB/GiB
symbols to make it a bit clearer.

While there we add a bit of clarity and rationale to the comment about
what happens if the LPCR[RMLS] doesn't contain a valid value.

Signed-off-by: David Gibson <[email protected]>
Reviewed-by: Cédric Le Goater <[email protected]>
Reviewed-by: Greg Kurz <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>