southbridge/hudson: Use MMIO instead of PIO to access PM space

The MMIO region is set up by AGESA very early on, so we can use it to
access the PM register space in ramstage. 16-bit accessors are also
provided to simplify some setup tasks. 16-bit accesses are not
possible via PIO.
The pm2_iowrite/read accessors are removed, as they are not used.

Change-Id: Ie7967b5086eb004525c39721338c6495aedc8165
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/5503
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@gmail.com>