tests: kernel: gen_isr_table: do not use IRQ 57 on NXP LPC55S16

IRQ 57 is reserved in the NXP LPC55S16 SoC. Thus, limit the number of
interrupts reported to the test, so that it does not try to use it.

Fixes #34915

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>