Split go_defaults into a 512 version and a normal version

Added go_defauls_common.mk for settings common to both 512 and other
Go devices. Added go_defaults_512 for 512MB RAM devices.

Bug: 67374015
Test: make, flash, look at properties
Change-Id: I160118e397a2f53d64df4a6d8e77aec3553fa401