x86: Don't build some unused objects in TPL

In the future if we have separate symbols for DM_SPI_FLASH and
SPL_DM_SPI_FLASH we will not always have function declarations available
for some DM calls.  This in turn leads to build warnings but not
failures as the code isn't used and is discarded at link time.
Restructure things to not build code we won't use for TPL anyways.

Cc: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
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I ran in to this as part of testing
http://patchwork.ozlabs.org/project/uboot/patch/20200604151153.3980-4-Zhiqiang.Hou@nxp.com/
which we do need to bring in.  I think we might be building a lot of
codde that's then discarded for TPL builds, but didn't think this is the
right place to tackle that in.