mmc: sdhci: change sdhci-pltfm into a module

There are a couple of problems left from the sdhci pltfm and OF
consolidation changes.

* When building more than one sdhci-pltfm based drivers in the same
  image, linker will give multiple definition error on the sdhci-pltfm
  helper functions.  For example right now, building sdhci-of-esdhc
  and sdhci-of-hlwd together is a valid combination from Kconfig view.

* With the current build method, there is error with building the
  drivers as module, but module installation fails with modprobe.

The patch fixes above problems by changing sdhci-pltfm into a module.
To avoid EXPORT_SYMBOL on so many big endian IO accessors, it moves
these accessors into sdhci-pltfm.h as the 'static inline' functions.
As a result, sdhci.h needs to be included in sdhci-pltfm.h, and in
turn can be removed from individual drivers which already include
sdhci-pltfm.h.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>