i82801gx: read RTC status register to prevent IRQ storm

i82801gx: read RTC status register to prevent IRQ storm

My Thinkpad appeared dead. After investigation, it turned out
that the RTC Alarm was triggering an RTC PM1 SMI, but the SMI
handler didn't read the status register, so it was triggered again.

This is a really nasty situation, as it means you have to dissemble
your Notebook just to unplug the RTC battery.

Change-Id: I5ac611e8a72deb5f38c86486dbe0693804935723
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Reviewed-on: http://review.coreboot.org/67
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>