target-arm: Fix decoding of preload and memory hint space

Correct the decoding of the ARM preload and memory hint space,
by adding decoding of PLI, PLDW and the v7MP unallocated hint
space. This commit also corrects a slightly overexuberant
decoding of PLD(register) which was not checking that bit 4
was one.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
