⚠️ Make PWM and XIRQ IOs fixed-sized

[stnolting]

This PR modifies the top entity's pwm_o and xirq_i ports. Both ports have now a fixed size that corresponds to the maximum value of the according configuration generics (rather than being size-defined by those generics). I think this is more straight-forward as all other IOs (except for the CFS ones) have a fixed size, too.

-- PWM (available if IO_PWM_NUM_CH > 0) --
pwm_o : out std_ulogic_vector(59 downto 0); -- pwm channels

-- External platform interrupts (available if XIRQ_NUM_CH > 0) --
xirq_i : in  std_ulogic_vector(31 downto 0) := (others => 'L'); -- IRQ channels