Fix AMD Agesa leaking Kconfig

Fix AMD Agesa leaking Kconfig

Kconfig leaked XIP_ROM_SIZE to other platforms and also
defined obsolete option XIP_ROM_BASE.

Alias AMD_AGESA as NORTHBRIDGE_AMD_AGESA.
Break the circular dependency with family15 Kconfig.

Change-Id: Ic7891012220e1bef758a5a39002b66971d5206e3
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/773
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>