PPC: BookE: Support 32 and 64 bit wide MAS2

The MAS registers on BookE are all 32 bit wide, except for MAS2, which
can hold up to 64 bit on 64 bit capable CPUs. Reflect this in the SPR
setting code, so that the guest can never write invalid values in them.

Signed-off-by: Alexander Graf <agraf@suse.de>
