arch: arm: nmi: simplify NMI initialization

Zephyr provides a default NMI handler (`z_SysNmiOnReset`), which will
basically call `wfi` endlessly. It is allowed to override such handler
when CONFIG_RUNTIME_NMI=y, via `z_arm_nmi_set_handler`. However,
enabling such option also provided `z_arm_nmi_init` (via `NMI_INIT()`),
which basically sets the handler to `DefaultHandler` (a new handler that
basically printks and reboots). This is strictly not needed, and
independent of the runtime NMI option. As a result, most SoCs were
blindly calling `NMI_INIT()`, probably because of a copy&paste effect.
In the majority of cases, this was a no-op, but most SoCs do IRQ
enable/disable, making this even more convoluted. To make things worse,
the init call is expected to run after console has been initialized (for
printk to work?), but most SoCs just called it in PRE_KERNEL_1+0.

This patch just drops this NMI initializer API, and leaves only the
handler set call when CONFIG_RUNTIME_NMI=y.

NMI_INIT() dummy definition is left in this patch to preserve
bisectability, will be dropped later.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>