x86: Cleanup linker scripts to use new DTS macros

Rework x86 linker scripts to use DT_REG_ADDR/DT_REG_SIZE on
DT_CHOSEN(zephyr_sram) and DT_CHOSEN(zephyr_flash).  As part of this
we remove the dts_fixup.h.  Using DT_REG_SIZE means we don't have to
adjust the sizes by 1024.

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