hw/intc/arm_gicv3_its: Rename max_l2_entries to num_l2_entries

In several places we have a local variable max_l2_entries which is
the number of entries which will fit in a level 2 table.  The
calculations done on this value are correct; rename it to
num_l2_entries to fit the convention we're using in this code.

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>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>