C6/H2: Enabling SYSTIMER_TARGET0 interrupt also enables UART0 interrupt

[bjoernQ]

This can be reproduced by running the systimer.rs example. Just run it and while running press any key ... There will be a panic because there isn't a UART0 interrupt handler

It's really just interrupt::enable(peripherals::Interrupt::SYSTIMER_TARGET0,Priority::Priority1) - another interrupt or using another priority won't trigger the problem

The problem most probably isn't a new one - we now just panic in case of an unhandled interrupt which makes the problem more visible.