sdhci-pltfm: reorganize Makefile entries to support SoC devices

Due to build system limitations, intermediate and final objects can't have
the same names.  And as we're going to start building SoC-specific
objects, let's rename the module to sdhci-platform, into which we'll link
sdhci-pltfm and SoC-specifc objects.

There should be no issue in renaming as the driver uses modalias
mechanism.

This is exactly the same approach as in sdhci-of driver.

Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>
Cc: Ben Dooks <ben@simtec.co.uk>
Cc: Richard R?jfors <richard.rojfors@pelagicore.com>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>