sdhci-of: rename main driver file prior to reorganization

This patch renames sdhci-of.c to sdhci-of-core.c before reorganizing the
driver to support additional hardware.

The driver is still built as sdhci-of despite the rename of the file.  No
functional change.

Signed-off-by: Albert Herranz <albert_herranz@yahoo.es>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>