hw/ppc: Avoid shifting left into sign bit

Add U suffix to various places where we were doing "1 << 31",
which is undefined behaviour, and also to other constant
definitions in the same groups, for consistency.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
