hw/intc/arm_gicv3_its: Remove maxids union from TableDesc

hw/intc/arm_gicv3_its: Remove maxids union from TableDesc

The TableDesc struct defines properties of the in-guest-memory tables
which the guest tells us about by writing to the GITS_BASER<n>
registers.  This struct currently has a union 'maxids', but all the
fields of the union have the same type (uint32_t) and do the same
thing (record one-greater-than the maximum ID value that can be used
as an index into the table).

We're about to add another table type (the GICv4 vPE table); rather
than adding another specifically-named union field for that table
type with the same type as the other union fields, remove the union
entirely and just have a 'uint32_t max_ids' struct field.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>