target-arm: Stop underdecoding ARM946 PRBS registers

The ARM946 has 8 PRBS (protection region base and size) registers.
Currently we implement these with a CP_ANY reginfo; however this
underdecodes (since there are 16 possible values of CRm but only
8 registers) and we catch the invalid values in the read and
write functions. However this causes issues with migration since
we only migrate the first of a wildcard register set, so we only
migrate c6_region[0]. It also makes it awkward to pull reginfo
access checks out into their own function.

Avoid all these problems by just defining separate reginfo structs
for each of the 8 registers; this also lets us avoid having any
read or write functions and will result in more efficient direct
field accesses from generated code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
