x86, geode-mfgpt: check IRQ before using MFGPT as clocksource

Adds a simple IRQ autodetection to the AMD Geode MFGPT driver, and more
importantly, adds some checks, if IRQs can actually be received on the
chosen line.  This fixes cases where MFGPT is selected as clocksource
though not producing any ticks, so the kernel simply starves during
boot.

Signed-off-by: Jens Rottmann <JRottmann@LiPPERTEmbedded.de>
Cc: Andres Salomon <dilinger@debian.org>
Cc: linux-geode@bombadil.infradead.org
Cc: Jordan Crouse <jordan.crouse@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>