boot: zephyr: Do not use `irq_lock()` if using arm cleanup

`irq_lock()` sets `BASEPRI_MAX` aka. the `BASEPRI` mask to whatever
zephyr has configured it to be by the value of `_EXC_IRQ_DEFAULT_PRIO`.

However by calling arm_cleanup() we also do the call to
`__disable_irq()` setting the PRIMASK to 1. Meaning the only exceptions
we can recive is fault exceptions. Masking out more exceptions does not
really make sense.

Sometimes applications booted by MCUBoot will not expect the `BASEPRI`
to be set to something else than 0(No effect). Meaning if they depend on
using some exception which now is masked out by `BASEPRI` they will
fail.

Signed-off-by: Sigvart Hovland <sigvart.hovland@nordicsemi.no>