davinci: mmc: pass number of SG segments as platform data

On some platforms like DM355, the number of EDMA parameter slots available
for EDMA_SLOT_ANY usage are few.  In such cases, if MMC/SD uses 16 slots
for each instance of MMC controller, then the number of slots available
for other modules will be very few.

By passing the number of EDMA slots to be used in MMC driver from platform
data, EDMA slots available for other purposes can be controlled.

Most of the platforms will not use this platform data variable.  But on
DM355, as the number of EDMA resources available is limited, the number of
scatter- gather segments used inside the MMC driver can be 8 (passed as
platform data) instead of 16.  On DM355, when the number of scatter-gather
segments was reduced to 8, I saw a performance difference of about
0.25-0.4 Mbytes/sec during write.  Read performance variations were
negligible.

Signed-off-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.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>