Fix computation of L1 bitmask in the translation table lib

Fix computation of L1 bitmask in the translation table lib

This patch fixes the computation of the bitmask used to isolate
the level 1 field of a virtual address. The whole computation needs
to work on 64-bit values to produce the correct bitmask value.
XLAT_TABLE_ENTRIES_MASK being a C constant, it is a 32-bit value
so it needs to be extended to a 64-bit value before it takes part
in any other computation.

This patch fixes this bug by casting XLAT_TABLE_ENTRIES_MASK as
an unsigned long long.

Note that this bug doesn't manifest itself in practice because
address spaces larger than 39 bits are not yet supported in the
Trusted Firmware.

Change-Id: I955fd263ecb691ca94b29b9c9f576008ce1d87ee