drivers/interrupt_controller: Get the IOAPIC RTEs dynamically

The fixed number of 24 RTEs is a legacy thing, and long gone by now.
IOAPICs expose the maximum number of RTEs they have via the version
register, so let's use it.

This avoids to manually tweak a Kconfig option (which is now removed)
and fixes the RTE number for all x86 targets relevantly.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>