ARCv2: Accomodate HS48 MMUv5 by relaxing MMU ver checking

HS48 cpus will have a new MMUv5, although Linux is currently not
explicitly supporting the newer features (so remains at V4).
The existing software/hardware version check is very tight and causes
boot abort. Given that the MMUv5 hardware is backwards compatible,
relax the boot check to allow current kernel support level to work
with new hardware.

Also while at it, move the ancient MMU related code to under ARCompact
builds as baseline MMU for HS cpus is v4.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>