SB800: Hide unused gpp ports

SB800: Hide unused gpp ports

Add configure option SB_GPP_UNHIDE_PORTS for mainboard
to hide/unhide the unused sb800 gpp ports.
Certain gpp port should be hidden, if no device was detected and
hotplug feature is disabled for such port.
Hidden unused ports makes lspci -vvv get more accurate information under Linux.
Test on avalue/eax-785e mainboard.

Change-Id: I1d7df0f2ab6ad69b1b99b8bf046411ae7cdb09c0
Signed-off-by: Kerry Sheh <kerry.she@amd.com>
Signed-off-by: Kerry Sheh <shekairui@gmail.com>
Reviewed-on: http://review.coreboot.org/207
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>