soc: x86: atom: sync linker / memory map on atom with ia32

Rework the atom/linker.ld to match how ia32/linker.ld works with regards
to how PHYS_LOAD_ADDR & PHYS_RAM_ADDR are set based on CONFIG_XIP and
from which DT_PHYS_{RAM,LOAD}_ADDR defines.  We update the minnowboard
dts and atom.dtsi files to keep in sync with this.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>