stm32: Use DT_NUM_INST_STATUS_OKAY for num IRQ sources

Use DT_NUM_INST_STATUS_OKAY(DT_DRV_COMPAT) to determine
number of potential interrupt sources. This mitigates
the wasting of memory on devices with mutliple UCPD ports
but only one UCPD port is used.

Signed-off-by: Sam Hurst <sbh1187@gmail.com>