booke_206_tlbwe: Discard invalid bits in MAS2

The size of EPN field in MAS2 depends on page size. This patch adds a
mask to discard invalid bits in EPN field.

Definition of EPN field from e500v2 RM:
EPN Effective page number: Depending on page size, only the bits
associated with a page boundary are valid. Bits that represent offsets
within a page are ignored and should be cleared.

There is a similar (but more complicated) definition in PowerISA V2.06.

Signed-off-by: Fabien Chouteau <chouteau@adacore.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
