hw/intc/arm_gicv3_its: Correct comment about CTE RDBase field size

hw/intc/arm_gicv3_its: Correct comment about CTE RDBase field size

The comment says that in our CTE format the RDBase field is 36 bits;
in fact for us it is only 16 bits, because we use the RDBase format
where it specifies a 16-bit CPU number. The code already uses
RDBASE_PROCNUM_LENGTH (16) as the field width, so fix the comment
to match it.

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>