mmc: sdhci-pci: Build o2micro support in the same module

sdhci-pci-o2micro.c contains no initialisation and its functions are only
called from shdci-pci.c, so there is no reason for it to be a separate
module, let alone or for it to always be built-in.

- Rename sdhci-pci.c to sdhci-pci-core.c so that the sdhci-pci module
  can be built from multiple source files
- Add sdhci-pci-o2micro.c to it
- Remove redundant exports

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>