memory: pl172: correct MPMC peripheral ID register bits

According to PL172 TRM read of bits [7:6] of MPMCPeriphID3 is
undefined, so unmask them. Also the driver supports all currently
present revisions of PL172, this allows to alleviate requirements to
the revision version matched by the driver.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Joachim Eastwood <manabian@gmail.com>