Intel northbridge I945: Apply un-written naming rules

Intel northbridge I945: Apply un-written naming rules

Use NORTHBRIDGE_INTEL_I945 to select the driver directory for build.

Use _SUBTYPE_945GC and _SUBTYPE_945GM to define at compile-time
which model of I945 the driver is built for.

Change-Id: I11b1e0998d0fc28f8946bad4f0989036a9b18af4
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/684
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>